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 51da6bb96cf1c55895857a36e438361c3e0e5640..f6b7428f247644f0866960e1eccf4d62f06f22f9 100644
--- a/content/renderer/media/media_stream_video_source.cc
+++ b/content/renderer/media/media_stream_video_source.cc
@@ -356,10 +356,13 @@ 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() {
@@ -385,6 +388,7 @@ webrtc::VideoSourceInterface* MediaStreamVideoSource::GetAdapter() {
 }
 
 void MediaStreamVideoSource::DoStopSource() {
+  DCHECK(CalledOnValidThread());
   DVLOG(3) << "DoStopSource()";
   StopSourceImpl();
   state_ = ENDED;