--- a/dom/media/ChannelMediaDecoder.h
+++ b/dom/media/ChannelMediaDecoder.h
@@ -62,18 +62,16 @@ protected:
                       MediaDecoderEventVisibility aEventVisibility) override;
 
   RefPtr<ResourceCallback> mResourceCallback;
   RefPtr<BaseMediaResource> mResource;
 
 public:
   explicit ChannelMediaDecoder(MediaDecoderInit& aInit);
 
-  MediaDecoderStateMachine* CreateStateMachine() override;
-
   MediaResource* GetResource() const override final;
 
   void Shutdown() override;
 
   bool CanClone();
 
   // Create a new decoder of the same type as this one.
   already_AddRefed<ChannelMediaDecoder> Clone(MediaDecoderInit& aInit);
@@ -82,16 +80,19 @@ public:
                 bool aIsPrivateBrowsing,
                 nsIStreamListener** aStreamListener);
 
   void SetLoadInBackground(bool aLoadInBackground) override;
   void Suspend() override;
   void Resume() override;
 
 private:
+  // Create a new state machine to run this decoder.
+  MediaDecoderStateMachine* CreateStateMachine();
+
   virtual ChannelMediaDecoder* CloneImpl(MediaDecoderInit& aInit) = 0;
   nsresult OpenResource(nsIStreamListener** aStreamListener);
   nsresult Load(BaseMediaResource* aOriginal);
 
   // Called by MediaResource when the download has ended.
   // Called on the main thread only. aStatus is the result from OnStopRequest.
   void NotifyDownloadEnded(nsresult aStatus);
 

--- a/dom/media/MediaDecoder.h
+++ b/dom/media/MediaDecoder.h
@@ -103,20 +103,16 @@ public:
   static void InitStatics();
 
   explicit MediaDecoder(MediaDecoderInit& aInit);
 
   // Returns the container content type of the resource.
   // Safe to call from any thread.
   const MediaContainerType& ContainerType() const { return mContainerType; }
 
-  // Create a new state machine to run this decoder.
-  // Subclasses must implement this.
-  virtual MediaDecoderStateMachine* CreateStateMachine() = 0;
-
   // Cleanup internal data structures. Must be called on the main
   // thread by the owning object before that object disposes of this object.
   virtual void Shutdown();
 
   // Notified by the shutdown manager that XPCOM shutdown has begun.
   // The decoder should notify its owner to drop the reference to the decoder
   // to prevent further calls into the decoder.
   void NotifyXPCOMShutdown();

--- a/dom/media/mediasource/MediaSourceDecoder.h
+++ b/dom/media/mediasource/MediaSourceDecoder.h
@@ -24,17 +24,16 @@ class MediaSource;
 
 class MediaSourceDecoder : public MediaDecoder
 {
 public:
   explicit MediaSourceDecoder(MediaDecoderInit& aInit);
 
   MediaResource* GetResource() const override final;
 
-  MediaDecoderStateMachine* CreateStateMachine() override;
   nsresult Load(nsIPrincipal* aPrincipal);
   media::TimeIntervals GetSeekable() override;
   media::TimeIntervals GetBuffered() override;
 
   void Shutdown() override;
 
   void AttachMediaSource(dom::MediaSource* aMediaSource);
   void DetachMediaSource();
@@ -60,16 +59,17 @@ public:
 
   MediaDecoderOwner::NextFrameStatus NextFrameBufferedStatus() override;
 
   bool IsMSE() const override { return true; }
 
   void NotifyInitDataArrived();
 
 private:
+  MediaDecoderStateMachine* CreateStateMachine();
   void DoSetMediaSourceDuration(double aDuration);
   media::TimeInterval ClampIntervalToEnd(const media::TimeInterval& aInterval);
   bool CanPlayThroughImpl() override;
 
   RefPtr<MediaSourceResource> mResource;
 
   // The owning MediaSource holds a strong reference to this decoder, and
   // calls Attach/DetachMediaSource on this decoder to set and clear