Bug 1348432 - move the mMediaTracksConstructed flag back to MediaDecoder; r=jwwang

This mMediaTracksConstructed flag should belong to a MediaDecoder,
every time a HTMLMediaElement switches its MediaDecoder, the flag should be reset to false again.

So, we move the mMediaTracksConstructed flag back to MediaDecoder, by this way,
HTMLMediaElement provides only the mechanism to construct and remove media tracks,
and MediaDecoder uses the flag, mMediaTracksConstructed, to provide policy.

MozReview-Commit-ID: L7mMAmLjQCy

dom/html/HTMLMediaElement.cpp

@@ -3723,7 +3723,6 @@ HTMLMediaElement::HTMLMediaElement(already_AddRefed<mozilla::dom::NodeInfo>& aNo
     mDefaultPlaybackStartPosition(0.0),
     mIsAudioTrackAudible(false),
     mHasSuspendTaint(false),
-    mMediaTracksConstructed(false),
     mVisibilityState(Visibility::UNTRACKED),
     mErrorSink(new ErrorSink(this)),
     mAudioChannelWrapper(new AudioChannelAgentCallback(this, mAudioChannel))
@@ -7423,11 +7422,7 @@ HTMLMediaElement::GetDocument() const
 void
 HTMLMediaElement::ConstructMediaTracks(const MediaInfo* aInfo)
 {
-  if (mMediaTracksConstructed || !aInfo) {
-    return;
-  }
-
-  mMediaTracksConstructed = true;
+  MOZ_ASSERT(aInfo);
 
   AudioTrackList* audioList = AudioTracks();
   if (audioList && aInfo->HasAudio()) {
@@ -7461,8 +7456,6 @@ HTMLMediaElement::RemoveMediaTracks()
   if (videoList) {
     videoList->RemoveTracks();
   }
-
-  mMediaTracksConstructed = false;
 }
 
 class MediaElementGMPCrashHelper : public GMPCrashHelper