Implement a source for remote video tracks.

content/renderer/media/media_stream_center.cc

Index: content/renderer/media/media_stream_center.cc
diff --git a/content/renderer/media/media_stream_center.cc b/content/renderer/media/media_stream_center.cc
index 3318fbda9131d3ed50d3d6a25aa73ad8dc62deb1..d7279c7d47046c3aaf99296949ff8cb001778a06 100644
--- a/content/renderer/media/media_stream_center.cc
+++ b/content/renderer/media/media_stream_center.cc
@@ -52,12 +52,7 @@ void CreateNativeVideoMediaStreamTrack(
   DCHECK_EQ(source.type(), blink::WebMediaStreamSource::TypeVideo);
   MediaStreamVideoSource* native_source =
       MediaStreamVideoSource::GetVideoSource(source);
-  if (!native_source) {
-    // TODO(perkj): Implement support for sources from
-    // remote MediaStreams.
-    NOTIMPLEMENTED();
-    return;
-  }
+  DCHECK(native_source);
   blink::WebMediaStreamTrack writable_track(track);
   writable_track.setExtraData(
       new MediaStreamVideoTrack(native_source, source.constraints(),