--- a/dom/media/eme/CDMProxy.h
+++ b/dom/media/eme/CDMProxy.h
@@ -76,31 +76,32 @@ protected:
 public:
 
   NS_INLINE_DECL_PURE_VIRTUAL_REFCOUNTING
 
   // Main thread only.
   CDMProxy(dom::MediaKeys* aKeys,
            const nsAString& aKeySystem,
            bool aDistinctiveIdentifierRequired,
-           bool aPersistentStateRequired)
+           bool aPersistentStateRequired,
+           nsIEventTarget* aMainThread)
     : mKeys(aKeys)
     , mKeySystem(aKeySystem)
     , mDistinctiveIdentifierRequired(aDistinctiveIdentifierRequired)
     , mPersistentStateRequired(aPersistentStateRequired)
+    , mMainThread(aMainThread)
   {}
 
   // Main thread only.
   // Loads the CDM corresponding to mKeySystem.
   // Calls MediaKeys::OnCDMCreated() when the CDM is created.
   virtual void Init(PromiseId aPromiseId,
                     const nsAString& aOrigin,
                     const nsAString& aTopLevelOrigin,
-                    const nsAString& aName,
-                    nsIEventTarget* aMainThread) = 0;
+                    const nsAString& aName) = 0;
 
   virtual void OnSetDecryptorId(uint32_t aId) {}
 
   // Main thread only.
   // Uses the CDM to create a key session.
   // Calls MediaKeys::OnSessionActivated() when session is created.
   // Assumes ownership of (Move()s) aInitData's contents.
   virtual void CreateSession(uint32_t aCreateSessionToken,
@@ -255,27 +256,27 @@ protected:
 
   // Our reference back to the MediaKeys object.
   // WARNING: This is a non-owning reference that is cleared by MediaKeys
   // destructor. only use on main thread, and always nullcheck before using!
   MainThreadOnlyRawPtr<dom::MediaKeys> mKeys;
 
   const nsString mKeySystem;
 
-  // The main thread associated with the root document. Must be set in Init().
-  nsCOMPtr<nsIEventTarget> mMainThread;
-
   // Onwer specified thread. e.g. Gecko Media Plugin thread.
   // All interactions with the out-of-process EME plugin must come from this thread.
   RefPtr<nsIThread> mOwnerThread;
 
   nsCString mNodeId;
 
   CDMCaps mCapabilites;
 
   const bool mDistinctiveIdentifierRequired;
   const bool mPersistentStateRequired;
+
+  // The main thread associated with the root document.
+  const nsCOMPtr<nsIEventTarget> mMainThread;
 };
 
 
 } // namespace mozilla
 
 #endif // CDMProxy_h_

--- a/dom/media/eme/MediaKeys.cpp
+++ b/dom/media/eme/MediaKeys.cpp
@@ -325,48 +325,48 @@ public:
     return (mMediaKeys && mMediaKeys->GetParentObject()) ?
       do_AddRef(mMediaKeys->GetParentObject()) : nullptr;
   }
 private:
   WeakPtr<MediaKeys> mMediaKeys;
 };
 
 already_AddRefed<CDMProxy>
-MediaKeys::CreateCDMProxy()
+MediaKeys::CreateCDMProxy(nsIEventTarget* aMainThread)
 {
   RefPtr<CDMProxy> proxy;
 #ifdef MOZ_WIDGET_ANDROID
   if (IsWidevineKeySystem(mKeySystem)) {
     proxy = new MediaDrmCDMProxy(this,
                                  mKeySystem,
                                  mConfig.mDistinctiveIdentifier == MediaKeysRequirement::Required,
-                                 mConfig.mPersistentState == MediaKeysRequirement::Required);
+                                 mConfig.mPersistentState == MediaKeysRequirement::Required,
+                                 aMainThread);
   } else
 #endif
   {
     proxy = new GMPCDMProxy(this,
                             mKeySystem,
                             new MediaKeysGMPCrashHelper(this),
                             mConfig.mDistinctiveIdentifier == MediaKeysRequirement::Required,
-                            mConfig.mPersistentState == MediaKeysRequirement::Required);
+                            mConfig.mPersistentState == MediaKeysRequirement::Required,
+                            aMainThread);
   }
   return proxy.forget();
 }
 
 already_AddRefed<DetailedPromise>
 MediaKeys::Init(ErrorResult& aRv)
 {
   RefPtr<DetailedPromise> promise(MakePromise(aRv,
     NS_LITERAL_CSTRING("MediaKeys::Init()")));
   if (aRv.Failed()) {
     return nullptr;
   }
 
-  mProxy = CreateCDMProxy();
-
   // Determine principal (at creation time) of the MediaKeys object.
   nsCOMPtr<nsIScriptObjectPrincipal> sop = do_QueryInterface(GetParentObject());
   if (!sop) {
     promise->MaybeReject(NS_ERROR_DOM_INVALID_STATE_ERR,
                          NS_LITERAL_CSTRING("Couldn't get script principal in MediaKeys::Init"));
     return promise.forget();
   }
   mPrincipal = sop->GetPrincipal();
@@ -410,32 +410,33 @@ MediaKeys::Init(ErrorResult& aRv)
     return promise.forget();
   }
 
   EME_LOG("MediaKeys[%p]::Create() (%s, %s)",
           this,
           origin.get(),
           topLevelOrigin.get());
 
+  mProxy = CreateCDMProxy(top->GetExtantDoc()->EventTargetFor(TaskCategory::Other));
+
   // The CDMProxy's initialization is asynchronous. The MediaKeys is
   // refcounted, and its instance is returned to JS by promise once
   // it's been initialized. No external refs exist to the MediaKeys while
   // we're waiting for the promise to be resolved, so we must hold a
   // reference to the new MediaKeys object until it's been created,
   // or its creation has failed. Store the id of the promise returned
   // here, and hold a self-reference until that promise is resolved or
   // rejected.
   MOZ_ASSERT(!mCreatePromiseId, "Should only be created once!");
   mCreatePromiseId = StorePromise(promise);
   AddRef();
   mProxy->Init(mCreatePromiseId,
                NS_ConvertUTF8toUTF16(origin),
                NS_ConvertUTF8toUTF16(topLevelOrigin),
-               KeySystemToGMPName(mKeySystem),
-               top->GetExtantDoc()->EventTargetFor(TaskCategory::Other));
+               KeySystemToGMPName(mKeySystem));
 
   return promise.forget();
 }
 
 void
 MediaKeys::OnCDMCreated(PromiseId aId, const uint32_t aPluginId)
 {
   RefPtr<DetailedPromise> promise(RetrievePromise(aId));

--- a/dom/media/eme/MediaKeys.h
+++ b/dom/media/eme/MediaKeys.h
@@ -129,17 +129,17 @@ public:
   bool IsBoundToMediaElement() const;
 
   void GetSessionsInfo(nsString& sessionsInfo);
 
 private:
 
   // Instantiate CDMProxy instance.
   // It could be MediaDrmCDMProxy (Widevine on Fennec) or GMPCDMProxy (the rest).
-  already_AddRefed<CDMProxy> CreateCDMProxy();
+  already_AddRefed<CDMProxy> CreateCDMProxy(nsIEventTarget* aMainThread);
 
   // Removes promise from mPromises, and returns it.
   already_AddRefed<DetailedPromise> RetrievePromise(PromiseId aId);
 
   // Owning ref to proxy. The proxy has a weak reference back to the MediaKeys,
   // and the MediaKeys destructor clears the proxy's reference to the MediaKeys.
   RefPtr<CDMProxy> mProxy;
 

--- a/dom/media/eme/mediadrm/MediaDrmCDMProxy.cpp
+++ b/dom/media/eme/mediadrm/MediaDrmCDMProxy.cpp
@@ -20,50 +20,49 @@ ToMediaDrmSessionType(dom::MediaKeySessi
     case dom::MediaKeySessionType::Persistent_license: return kKeyOffline;
     default: return kKeyStreaming;
   };
 }
 
 MediaDrmCDMProxy::MediaDrmCDMProxy(dom::MediaKeys* aKeys,
                                    const nsAString& aKeySystem,
                                    bool aDistinctiveIdentifierRequired,
-                                   bool aPersistentStateRequired)
+                                   bool aPersistentStateRequired,
+                                   nsIEventTarget* aMainThread)
   : CDMProxy(aKeys,
              aKeySystem,
              aDistinctiveIdentifierRequired,
-             aPersistentStateRequired)
+             aPersistentStateRequired,
+             aMainThread)
   , mCDM(nullptr)
   , mShutdownCalled(false)
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_COUNT_CTOR(MediaDrmCDMProxy);
 }
 
 MediaDrmCDMProxy::~MediaDrmCDMProxy()
 {
   MOZ_COUNT_DTOR(MediaDrmCDMProxy);
 }
 
 void
 MediaDrmCDMProxy::Init(PromiseId aPromiseId,
                        const nsAString& aOrigin,
                        const nsAString& aTopLevelOrigin,
-                       const nsAString& aName,
-                       nsIEventTarget* aMainThread)
+                       const nsAString& aName)
 {
   MOZ_ASSERT(NS_IsMainThread());
   NS_ENSURE_TRUE_VOID(!mKeys.IsNull());
 
   EME_LOG("MediaDrmCDMProxy::Init (%s, %s) %s",
           NS_ConvertUTF16toUTF8(aOrigin).get(),
           NS_ConvertUTF16toUTF8(aTopLevelOrigin).get(),
           NS_ConvertUTF16toUTF8(aName).get());
 
-  mMainThread = aMainThread;
-
   // Create a thread to work with cdm.
   if (!mOwnerThread) {
     nsresult rv = NS_NewNamedThread("MDCDMThread", getter_AddRefs(mOwnerThread));
     if (NS_FAILED(rv)) {
       RejectPromise(aPromiseId, NS_ERROR_DOM_INVALID_STATE_ERR,
                     NS_LITERAL_CSTRING("Couldn't create CDM thread MediaDrmCDMProxy::Init"));
       return;
     }

--- a/dom/media/eme/mediadrm/MediaDrmCDMProxy.h
+++ b/dom/media/eme/mediadrm/MediaDrmCDMProxy.h
@@ -27,23 +27,23 @@ class MediaDrmCDMCallbackProxy;
 class MediaDrmCDMProxy : public CDMProxy {
 public:
 
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MediaDrmCDMProxy)
 
   MediaDrmCDMProxy(dom::MediaKeys* aKeys,
                    const nsAString& aKeySystem,
                    bool aDistinctiveIdentifierRequired,
-                   bool aPersistentStateRequired);
+                   bool aPersistentStateRequired,
+                   nsIEventTarget* aMainThread);
 
   void Init(PromiseId aPromiseId,
             const nsAString& aOrigin,
             const nsAString& aTopLevelOrigin,
-            const nsAString& aGMPName,
-            nsIEventTarget* aMainThread) override;
+            const nsAString& aGMPName) override;
 
   void CreateSession(uint32_t aCreateSessionToken,
                      MediaKeySessionType aSessionType,
                      PromiseId aPromiseId,
                      const nsAString& aInitDataType,
                      nsTArray<uint8_t>& aInitData) override;
 
   void LoadSession(PromiseId aPromiseId,

--- a/dom/media/gmp/GMPCDMProxy.cpp
+++ b/dom/media/gmp/GMPCDMProxy.cpp
@@ -23,21 +23,23 @@
 #include "GMPUtils.h"
 
 namespace mozilla {
 
 GMPCDMProxy::GMPCDMProxy(dom::MediaKeys* aKeys,
                          const nsAString& aKeySystem,
                          GMPCrashHelper* aCrashHelper,
                          bool aDistinctiveIdentifierRequired,
-                         bool aPersistentStateRequired)
+                         bool aPersistentStateRequired,
+                         nsIEventTarget* aMainThread)
   : CDMProxy(aKeys,
              aKeySystem,
              aDistinctiveIdentifierRequired,
-             aPersistentStateRequired)
+             aPersistentStateRequired,
+             aMainThread)
   , mCrashHelper(aCrashHelper)
   , mCDM(nullptr)
   , mShutdownCalled(false)
   , mDecryptorId(0)
   , mCreatePromiseId(0)
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_COUNT_CTOR(GMPCDMProxy);
@@ -47,28 +49,25 @@ GMPCDMProxy::~GMPCDMProxy()
 {
   MOZ_COUNT_DTOR(GMPCDMProxy);
 }
 
 void
 GMPCDMProxy::Init(PromiseId aPromiseId,
                   const nsAString& aOrigin,
                   const nsAString& aTopLevelOrigin,
-                  const nsAString& aGMPName,
-                  nsIEventTarget* aMainThread)
+                  const nsAString& aGMPName)
 {
   MOZ_ASSERT(NS_IsMainThread());
   NS_ENSURE_TRUE_VOID(!mKeys.IsNull());
 
   EME_LOG("GMPCDMProxy::Init (%s, %s)",
           NS_ConvertUTF16toUTF8(aOrigin).get(),
           NS_ConvertUTF16toUTF8(aTopLevelOrigin).get());
 
-  mMainThread = aMainThread;
-
   nsCString pluginVersion;
   if (!mOwnerThread) {
     nsCOMPtr<mozIGeckoMediaPluginService> mps =
       do_GetService("@mozilla.org/gecko-media-plugin-service;1");
     if (!mps) {
       RejectPromise(aPromiseId, NS_ERROR_DOM_INVALID_STATE_ERR,
                     NS_LITERAL_CSTRING("Couldn't get MediaPluginService in GMPCDMProxy::Init"));
       return;

--- a/dom/media/gmp/GMPCDMProxy.h
+++ b/dom/media/gmp/GMPCDMProxy.h
@@ -21,23 +21,23 @@ class GMPCDMProxy : public CDMProxy {
 public:
 
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(GMPCDMProxy, override)
 
   GMPCDMProxy(dom::MediaKeys* aKeys,
               const nsAString& aKeySystem,
               GMPCrashHelper* aCrashHelper,
               bool aDistinctiveIdentifierRequired,
-              bool aPersistentStateRequired);
+              bool aPersistentStateRequired,
+              nsIEventTarget* aMainThread);
 
   void Init(PromiseId aPromiseId,
             const nsAString& aOrigin,
             const nsAString& aTopLevelOrigin,
-            const nsAString& aGMPName,
-            nsIEventTarget* aMainThread) override;
+            const nsAString& aGMPName) override;
 
   void OnSetDecryptorId(uint32_t aId) override;
 
   void CreateSession(uint32_t aCreateSessionToken,
                      dom::MediaKeySessionType aSessionType,
                      PromiseId aPromiseId,
                      const nsAString& aInitDataType,
                      nsTArray<uint8_t>& aInitData) override;