Use base::Optional for selected video track.

media/filters/ffmpeg_demuxer.h

Index: media/filters/ffmpeg_demuxer.h
diff --git a/media/filters/ffmpeg_demuxer.h b/media/filters/ffmpeg_demuxer.h
index 84a8051e234975523382deaf8e16a65bac3f8769..a5b15bc6d9618095c4be3a8507136b927babc519 100644
--- a/media/filters/ffmpeg_demuxer.h
+++ b/media/filters/ffmpeg_demuxer.h
@@ -239,10 +239,11 @@ class MEDIA_EXPORT FFmpegDemuxer : public Demuxer {
   void NotifyDemuxerError(PipelineStatus error);
 
   void OnEnabledAudioTracksChanged(const std::vector<MediaTrack::Id>& track_ids,
-                                   base::TimeDelta currTime) override;
-  // |track_ids| is either empty or contains a single video track id.
-  void OnSelectedVideoTrackChanged(const std::vector<MediaTrack::Id>& track_ids,
-                                   base::TimeDelta currTime) override;
+                                   base::TimeDelta curr_time) override;
+  // |track_id| either contains the selected video track id or is null,
+  // indicating that all video tracks are deselected/disabled.
+  void OnSelectedVideoTrackChanged(base::Optional<MediaTrack::Id> track_id,
+                                   base::TimeDelta curr_time) override;
 
   // The lowest demuxed timestamp.  If negative, DemuxerStreams must use this to
   // adjust packet timestamps such that external clients see a zero-based