Bug 1156472 - Part 1 - Allow to capture all HTMLMediaElements and AudioContexts for a document. r=baku,padenot

This is built on top of the AudioChannel infrastructure. This patch does not
actually implement the capture, it just does the plumbing to be able to notify
all HTMLMediaElement/AudioContext for a document.

dom/html/HTMLMediaElement.cpp

@@ -4492,7 +4492,7 @@ void HTMLMediaElement::UpdateAudioChannelPlayingState()
       if (!mAudioChannelAgent) {
         return;
       }
-      mAudioChannelAgent->InitWithWeakCallback(OwnerDoc()->GetWindow(),
+      mAudioChannelAgent->InitWithWeakCallback(OwnerDoc()->GetInnerWindow(),
                                                static_cast<int32_t>(mAudioChannel),
                                                this);
     }
@@ -4504,6 +4504,10 @@ void HTMLMediaElement::UpdateAudioChannelPlayingState()
 void
 HTMLMediaElement::NotifyAudioChannelAgent(bool aPlaying)
 {
+    // Immediately check if this should go to the MSG instead of the normal
+    // media playback route.
+    WindowAudioCaptureChanged();
+
   // This is needed to pass nsContentUtils::IsCallerChrome().
   // AudioChannel API should not called from content but it can happen that
   // this method has some content JS in its stack.
@@ -4675,6 +4679,15 @@ HTMLMediaElement::GetTopLevelPrincipal()
 }
 #endif // MOZ_EME
 
+NS_IMETHODIMP HTMLMediaElement::WindowAudioCaptureChanged()
+{
+  MOZ_ASSERT(mAudioChannelAgent);
+  DebugOnly<bool> captured = OwnerDoc()->GetInnerWindow()->GetAudioCaptured();
+
+  // Something is going to happen here!!
+  return NS_OK;
+}
+
 AudioTrackList*
 HTMLMediaElement::AudioTracks()
 {