Bug 1172394 - Make dom::MediaTrack lifetime spec compliant. r=bryce

This makes us forget tracks at the right times. The spec also says no
removetrack events should be fired because of this, yet it seems to be something
other user agents do:
https://wpt.fyi/results/media-source/mediasource-avtracks.html

This is of low importance however, since MediaTracks are prefed off by default.

Differential Revision: https://phabricator.services.mozilla.com/D52038

dom/html/HTMLMediaElement.cpp

@@ -1917,17 +1917,6 @@ void HTMLMediaElement::AbortExistingLoads() {
     mFirstFrameListener = nullptr;
   }
 
-  // When aborting the existing loads, empty the objects in audio track list and
-  // video track list, no events (in particular, no removetrack events) are
-  // fired as part of this. Ending MediaTrack sends track ended notifications,
-  // so we empty the track lists prior.
-  if (AudioTracks()) {
-    AudioTracks()->EmptyTracks();
-  }
-  if (VideoTracks()) {
-    VideoTracks()->EmptyTracks();
-  }
-
   if (mDecoder) {
     fireTimeUpdate = mDecoder->GetCurrentTime() != 0.0;
     ShutdownDecoder();
@@ -1979,6 +1968,7 @@ void HTMLMediaElement::AbortExistingLoads() {
       RejectPromises(TakePendingPlayPromises(), NS_ERROR_DOM_MEDIA_ABORT_ERR);
     }
     ChangeNetworkState(NETWORK_EMPTY);
+    RemoveMediaTracks();
     ChangeReadyState(HAVE_NOTHING);
 
     // TODO: Apply the rules for text track cue rendering Bug 865407
@@ -2026,6 +2016,7 @@ void HTMLMediaElement::NoSupportedMediaSourceError(
     ShutdownDecoder();
   }
   mErrorSink->SetError(MEDIA_ERR_SRC_NOT_SUPPORTED, aErrorDetails);
+  RemoveMediaTracks();
   ChangeDelayLoadStatus(false);
   UpdateAudioChannelPlayingState();
   RejectPromises(TakePendingPlayPromises(),
@@ -2471,6 +2462,8 @@ void HTMLMediaElement::LoadFromSourceChildren() {
 
   AddMutationObserverUnlessExists(this);
 
+  RemoveMediaTracks();
+
   while (true) {
     Element* child = GetNextSource();
     if (!child) {
@@ -5119,6 +5112,10 @@ void HTMLMediaElement::MetadataLoaded(const MediaInfo* aInfo,
                                       UniquePtr<const MetadataTags> aTags) {
   MOZ_ASSERT(NS_IsMainThread());
 
+  if (mDecoder) {
+    ConstructMediaTracks(aInfo);
+  }
+
   SetMediaInfo(*aInfo);
 
   mIsEncrypted =
@@ -5234,12 +5231,6 @@ void HTMLMediaElement::DecodeError(const MediaResult& aError) {
   DecoderDoctorDiagnostics diagnostics;
   diagnostics.StoreDecodeError(OwnerDoc(), aError, src, __func__);
 
-  if (AudioTracks()) {
-    AudioTracks()->EmptyTracks();
-  }
-  if (VideoTracks()) {
-    VideoTracks()->EmptyTracks();
-  }
   if (mIsLoadingFromSourceChildren) {
     mErrorSink->ResetError();
     if (mSourceLoadCandidate) {
@@ -7241,12 +7232,10 @@ bool HTMLMediaElement::IsAudible() const {
 }
 
 void HTMLMediaElement::ConstructMediaTracks(const MediaInfo* aInfo) {
-  if (mMediaTracksConstructed || !aInfo) {
+  if (!aInfo) {
     return;
   }
 
-  mMediaTracksConstructed = true;
-
   AudioTrackList* audioList = AudioTracks();
   if (audioList && aInfo->HasAudio()) {
     const TrackInfo& info = aInfo->mAudio;
@@ -7273,12 +7262,9 @@ void HTMLMediaElement::RemoveMediaTracks() {
   if (mAudioTrackList) {
     mAudioTrackList->RemoveTracks();
   }
-
   if (mVideoTrackList) {
     mVideoTrackList->RemoveTracks();
   }
-
-  mMediaTracksConstructed = false;
 }
 
 class MediaElementGMPCrashHelper : public GMPCrashHelper {