Update MediaStreamTrack::Stop to latest draft.

content/renderer/media/media_stream_video_source.cc

Index: content/renderer/media/media_stream_video_source.cc
diff --git a/content/renderer/media/media_stream_video_source.cc b/content/renderer/media/media_stream_video_source.cc
index 243d361323c6a4116526437ea1462f61cb74fa6d..be6b23b2d9295888ca98a1a86bfefc5b37b38a3f 100644
--- a/content/renderer/media/media_stream_video_source.cc
+++ b/content/renderer/media/media_stream_video_source.cc
@@ -337,10 +337,13 @@ void MediaStreamVideoSource::AddTrack(
 }
 
 void MediaStreamVideoSource::RemoveTrack(MediaStreamVideoTrack* video_track) {
+  DCHECK(CalledOnValidThread());
   std::vector<MediaStreamVideoTrack*>::iterator it =
       std::find(tracks_.begin(), tracks_.end(), video_track);
   DCHECK(it != tracks_.end());
   tracks_.erase(it);
+  if (tracks_.empty())
+    StopSource();
 }
 
 void MediaStreamVideoSource::InitAdapter() {
@@ -367,6 +370,7 @@ webrtc::VideoSourceInterface* MediaStreamVideoSource::GetAdapter() {
 }
 
 void MediaStreamVideoSource::DoStopSource() {
+  DCHECK(CalledOnValidThread());
   DVLOG(3) << "DoStopSource()";
   StopSourceImpl();
   state_ = ENDED;