Bug 1309162 - part3 : modify the place calling UpdateAudioChannelPlayingState(). r=baku

UpdateAudioChannelPlayingState() should only be called when following attributes changed.
eg. pause/ready state/error/owner document's visibility/exteral source stream.

Therefore, we don't need to call this function in FinishDecoderSetup() and AddRemoveSelfReference().

Remove AutoNotifyAudioChannelAgent is because now we don't check HasAudio() for IsPlayingThroughTheAudioChannel().

MozReview-Commit-ID: 4VTHIAdAqX1

dom/html/HTMLMediaElement.cpp

@@ -123,24 +123,6 @@ static mozilla::LazyLogModule gMediaElementEventsLog("nsMediaElementEvents");
 using namespace mozilla::layers;
 using mozilla::net::nsMediaFragmentURIParser;
 
-class MOZ_STACK_CLASS AutoNotifyAudioChannelAgent
-{
-  RefPtr<mozilla::dom::HTMLMediaElement> mElement;
-  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER;
-public:
-  explicit AutoNotifyAudioChannelAgent(mozilla::dom::HTMLMediaElement* aElement
-                                       MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
-    : mElement(aElement)
-  {
-    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
-  }
-
-  ~AutoNotifyAudioChannelAgent()
-  {
-    mElement->UpdateAudioChannelPlayingState();
-  }
-};
-
 namespace mozilla {
 namespace dom {
 
@@ -4441,7 +4423,6 @@ nsresult HTMLMediaElement::FinishDecoderSetup(MediaDecoder* aDecoder,
   // We may want to suspend the new stream now.
   // This will also do an AddRemoveSelfReference.
   NotifyOwnerDocumentActivityChanged();
-  UpdateAudioChannelPlayingState();
 
   if (!mPaused) {
     SetPlayedOrSeeked(true);
@@ -4875,11 +4856,6 @@ void HTMLMediaElement::MetadataLoaded(const MediaInfo* aInfo,
 {
   MOZ_ASSERT(NS_IsMainThread());
 
-  // If the element is gaining or losing an audio track, we need to notify
-  // the audio channel agent so that the correct audio-playback events will
-  // get dispatched.
-  AutoNotifyAudioChannelAgent autoNotify(this);
-
   SetMediaInfo(*aInfo);
 
   mIsEncrypted = aInfo->IsEncrypted() || mPendingEncryptedInitData.IsEncrypted();
@@ -5946,8 +5922,6 @@ void HTMLMediaElement::AddRemoveSelfReference()
       NS_DispatchToMainThread(event);
     }
   }
-
-  UpdateAudioChannelPlayingState();
 }
 
 void HTMLMediaElement::DoRemoveSelfReference()