We have recently finished adding basic support for media tracks in HTMLMediaElement, demuxers and media::RendererImpl (see  crbug.com/249427 ). Now we'll need to add media track support also in mojo (probably by moving the media track-supporting code from media::Demuxer into media::DemuxerStreamProvider interface, which is common between mojo and current media stack). And then we'll also need to handle media track status changes (OnEnabledAudioStreamsChanged and OnSelectedVideoStreamChanged) in mojo media renderer.