MediaStreamVideoTrack: survive source getting deallocated.

This change fixes a problem where MediaStreamVideoTrack would would CHECK-fail
when the source has been cleared or deallocated AND a call to either SetEnabled,
AddEncodedSink or RemoveEncodedSink occurs.

The change also includes a new unit test for this case which was previously
uncovered.

Change-Id: Iab02df69277b2d2a287e2505977ce94345afb848
Bug: 1013590
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993291Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Markus Handell <handellm@google.com>
Cr-Commit-Position: refs/heads/master@{#730138}

third_party/blink/renderer/modules/mediastream/media_stream_video_track.cc

@@ -494,7 +494,8 @@ void MediaStreamVideoTrack::AddEncodedSink(WebMediaStreamSink* sink,
   DCHECK_CALLED_ON_VALID_THREAD(main_render_thread_checker_);
   AddSinkInternal(&encoded_sinks_, sink);
   frame_deliverer_->AddEncodedCallback(sink, std::move(callback));
-  source_->UpdateNumEncodedSinks();
+  if (source_)
+    source_->UpdateNumEncodedSinks();
   UpdateSourceHasConsumers();
 }
 
@@ -514,7 +515,8 @@ void MediaStreamVideoTrack::RemoveEncodedSink(WebMediaStreamSink* sink) {
   DCHECK_CALLED_ON_VALID_THREAD(main_render_thread_checker_);
   RemoveSinkInternal(&encoded_sinks_, sink);
   frame_deliverer_->RemoveEncodedCallback(sink);
-  source_->UpdateNumEncodedSinks();
+  if (source_)
+    source_->UpdateNumEncodedSinks();
   UpdateSourceHasConsumers();
 }
 
@@ -532,7 +534,8 @@ void MediaStreamVideoTrack::SetEnabled(bool enabled) {
   // need a new keyframe from the source as we may have dropped data making the
   // stream undecodable.
   bool maybe_await_key_frame = false;
-  if (enabled && source_->SupportsEncodedOutput() && !encoded_sinks_.empty()) {
+  if (enabled && source_ && source_->SupportsEncodedOutput() &&
+      !encoded_sinks_.empty()) {
     source_->RequestRefreshFrame();
     maybe_await_key_frame = true;
   }

third_party/blink/renderer/modules/mediastream/media_stream_video_track_test.cc

@@ -255,6 +255,22 @@ TEST_F(MediaStreamVideoTrackTest, SetEnabled) {
   sink.DisconnectFromTrack();
 }
 
+TEST_F(MediaStreamVideoTrackTest, SourceDetached) {
+  InitializeSource();
+  WebMediaStreamTrack track = CreateTrack();
+  MockMediaStreamVideoSink sink;
+  auto* video_track = MediaStreamVideoTrack::GetVideoTrack(track);
+  video_track->StopAndNotify(base::BindOnce([] {}));
+  sink.ConnectToTrack(track);
+  sink.ConnectEncodedToTrack(track);
+  video_track->SetEnabled(true);
+  video_track->SetEnabled(false);
+  WebMediaStreamTrack::Settings settings;
+  video_track->GetSettings(settings);
+  sink.DisconnectFromTrack();
+  sink.DisconnectEncodedFromTrack();
+}
+
 TEST_F(MediaStreamVideoTrackTest, SourceStopped) {
   InitializeSource();
   MockMediaStreamVideoSink sink;