--- a/dom/media/DOMMediaStream.cpp
+++ b/dom/media/DOMMediaStream.cpp
@@ -788,23 +788,25 @@ DOMMediaStream::NotifyPrincipalChanged()
 {
   for (uint32_t i = 0; i < mPrincipalChangeObservers.Length(); ++i) {
     mPrincipalChangeObservers[i]->PrincipalChanged(this);
   }
 }
 
 
 bool
-DOMMediaStream::AddPrincipalChangeObserver(PrincipalChangeObserver* aObserver)
+DOMMediaStream::AddPrincipalChangeObserver(
+  PrincipalChangeObserver<DOMMediaStream>* aObserver)
 {
   return mPrincipalChangeObservers.AppendElement(aObserver) != nullptr;
 }
 
 bool
-DOMMediaStream::RemovePrincipalChangeObserver(PrincipalChangeObserver* aObserver)
+DOMMediaStream::RemovePrincipalChangeObserver(
+  PrincipalChangeObserver<DOMMediaStream>* aObserver)
 {
   return mPrincipalChangeObservers.RemoveElement(aObserver);
 }
 
 MediaStreamTrack*
 DOMMediaStream::CreateOwnDOMTrack(TrackID aTrackID, MediaSegment::Type aType,
                                   const nsString& aLabel,
                                   MediaStreamTrackSource* aSource)

--- a/dom/media/DOMMediaStream.h
+++ b/dom/media/DOMMediaStream.h
@@ -11,16 +11,17 @@
 #include "nsCycleCollectionParticipant.h"
 #include "nsWrapperCache.h"
 #include "StreamBuffer.h"
 #include "nsIDOMWindow.h"
 #include "nsIPrincipal.h"
 #include "mozilla/PeerIdentity.h"
 #include "mozilla/DOMEventTargetHelper.h"
 #include "mozilla/CORSMode.h"
+#include "PrincipalChangeObserver.h"
 
 // GetCurrentTime is defined in winbase.h as zero argument macro forwarding to
 // GetTickCount() and conflicts with NS_DECL_NSIDOMMEDIASTREAM, containing
 // currentTime getter.
 #ifdef GetCurrentTime
 #undef GetCurrentTime
 #endif
 // X11 has a #define for CurrentTime. Unbelievable :-(.
@@ -451,27 +452,18 @@ public:
 
   /**
    * This is used in WebRTC to move from a protected state (nsNullPrincipal) to
    * one where the stream is accessible to script.  Don't call this.
    * CombineWithPrincipal is almost certainly more appropriate.
    */
   void SetPrincipal(nsIPrincipal* aPrincipal);
 
-  /**
-   * Used to learn about dynamic changes in principal occur.
-   * Operations relating to these observers must be confined to the main thread.
-   */
-  class PrincipalChangeObserver
-  {
-  public:
-    virtual void PrincipalChanged(DOMMediaStream* aMediaStream) = 0;
-  };
-  bool AddPrincipalChangeObserver(PrincipalChangeObserver* aObserver);
-  bool RemovePrincipalChangeObserver(PrincipalChangeObserver* aObserver);
+  bool AddPrincipalChangeObserver(dom::PrincipalChangeObserver<DOMMediaStream>* aObserver);
+  bool RemovePrincipalChangeObserver(dom::PrincipalChangeObserver<DOMMediaStream>* aObserver);
 
   /**
    * Called when this stream's MediaStreamGraph has been shut down. Normally
    * MSGs are only shut down when all streams have been removed, so this
    * will only be called during a forced shutdown due to application exit.
    */
   void NotifyMediaStreamGraphShutdown();
   /**
@@ -647,17 +639,17 @@ protected:
   nsTArray<RefPtr<TrackListener>> mTrackListeners;
 
 private:
   void NotifyPrincipalChanged();
 
   // Principal identifying who may access the contents of this stream.
   // If null, this stream can be used by anyone because it has no content yet.
   nsCOMPtr<nsIPrincipal> mPrincipal;
-  nsTArray<PrincipalChangeObserver*> mPrincipalChangeObservers;
+  nsTArray<dom::PrincipalChangeObserver<DOMMediaStream>*> mPrincipalChangeObservers;
   // this is used in gUM and WebRTC to identify peers that this stream
   // is allowed to be sent to
   nsAutoPtr<PeerIdentity> mPeerIdentity;
   CORSMode mCORSMode;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(DOMMediaStream,
                               NS_DOMMEDIASTREAM_IID)

--- a/dom/media/imagecapture/CaptureTask.h
+++ b/dom/media/imagecapture/CaptureTask.h
@@ -23,31 +23,31 @@ class ImageCapture;
  * class into MediaStream and retrieves an image in MediaStreamGraph thread.
  * Once the image is retrieved, it will be sent to ImageEncoder and the encoded
  * blob will be sent out via encoder callback in main thread.
  *
  * CaptureTask holds a reference of ImageCapture to ensure ImageCapture won't be
  * released during the period of the capturing process described above.
  */
 class CaptureTask : public MediaStreamListener,
-                    public DOMMediaStream::PrincipalChangeObserver
+                    public dom::PrincipalChangeObserver<DOMMediaStream>
 {
 public:
   // MediaStreamListener methods.
   void NotifyQueuedTrackChanges(MediaStreamGraph* aGraph, TrackID aID,
                                 StreamTime aTrackOffset,
                                 uint32_t aTrackEvents,
                                 const MediaSegment& aQueuedMedia,
                                 MediaStream* aInputStream,
                                 TrackID aInputTrackID) override;
 
   void NotifyEvent(MediaStreamGraph* aGraph,
                    MediaStreamGraphEvent aEvent) override;
 
-  // DOMMediaStream::PrincipalChangeObserver method.
+  // PrincipalChangeObserver<DOMMediaStream> method.
   void PrincipalChanged(DOMMediaStream* aMediaStream) override;
 
   // CaptureTask methods.
 
   // It is called when aBlob is ready to post back to script in company with
   // aRv == NS_OK. If aRv is not NS_OK, it will post an error event to script.
   //
   // Note:

--- a/dom/media/imagecapture/ImageCapture.cpp
+++ b/dom/media/imagecapture/ImageCapture.cpp
@@ -73,17 +73,17 @@ ImageCapture::GetVideoStreamTrack() cons
 }
 
 nsresult
 ImageCapture::TakePhotoByMediaEngine()
 {
   // Callback for TakPhoto(), it also monitor the principal. If principal
   // changes, it returns PHOTO_ERROR with security error.
   class TakePhotoCallback : public MediaEnginePhotoCallback,
-                            public DOMMediaStream::PrincipalChangeObserver
+                            public PrincipalChangeObserver<DOMMediaStream>
   {
   public:
     TakePhotoCallback(VideoStreamTrack* aVideoTrack, ImageCapture* aImageCapture)
       : mVideoTrack(aVideoTrack)
       , mImageCapture(aImageCapture)
       , mPrincipalChanged(false)
     {
       MOZ_ASSERT(NS_IsMainThread());

--- a/dom/media/webaudio/MediaStreamAudioSourceNode.h
+++ b/dom/media/webaudio/MediaStreamAudioSourceNode.h
@@ -36,17 +36,17 @@ public:
     }
   }
 
 private:
   bool mEnabled;
 };
 
 class MediaStreamAudioSourceNode : public AudioNode,
-                                   public DOMMediaStream::PrincipalChangeObserver
+                                   public PrincipalChangeObserver<DOMMediaStream>
 {
 public:
   static already_AddRefed<MediaStreamAudioSourceNode>
   Create(AudioContext* aContext, DOMMediaStream* aStream, ErrorResult& aRv);
 
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(MediaStreamAudioSourceNode, AudioNode)
 

--- a/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.h
+++ b/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.h
@@ -244,17 +244,17 @@ class RTCStatsQuery {
       nsresult res = CheckApiState(assert_ice_ready);             \
       if (NS_FAILED(res)) return; \
     } while(0)
 #define PC_AUTO_ENTER_API_CALL_NO_CHECK() CheckThread()
 
 class PeerConnectionImpl final : public nsISupports,
 #if !defined(MOZILLA_EXTERNAL_LINKAGE)
                                  public mozilla::DataChannelConnection::DataConnectionListener,
-                                 public DOMMediaStream::PrincipalChangeObserver,
+                                 public dom::PrincipalChangeObserver<DOMMediaStream>,
 #endif
                                  public sigslot::has_slots<>
 {
   struct Internal; // Avoid exposing c includes to bindings
 
 public:
   explicit PeerConnectionImpl(const mozilla::dom::GlobalObject* aGlobal = nullptr);