Use base::Optional for selected video track.

media/filters/ffmpeg_demuxer.cc

Index: media/filters/ffmpeg_demuxer.cc
diff --git a/media/filters/ffmpeg_demuxer.cc b/media/filters/ffmpeg_demuxer.cc
index d7146034bbf45e52c1872f8fb113d40bf4366d10..43a8b2c5c4916adee4623c4120191d18e88750f0 100644
--- a/media/filters/ffmpeg_demuxer.cc
+++ b/media/filters/ffmpeg_demuxer.cc
@@ -1662,14 +1662,13 @@ void FFmpegDemuxer::OnEnabledAudioTracksChanged(
 }
 
 void FFmpegDemuxer::OnSelectedVideoTrackChanged(
-    const std::vector<MediaTrack::Id>& track_ids,
+    base::Optional<MediaTrack::Id> track_id,
     base::TimeDelta currTime) {
   DCHECK(task_runner_->BelongsToCurrentThread());
-  DCHECK_LE(track_ids.size(), 1u);
 
   FFmpegDemuxerStream* selected_stream = nullptr;
-  if (!track_ids.empty()) {
-    selected_stream = track_id_to_demux_stream_map_[track_ids[0]];
+  if (track_id) {
+    selected_stream = track_id_to_demux_stream_map_[*track_id];
     DCHECK(selected_stream);
     DCHECK_EQ(DemuxerStream::VIDEO, selected_stream->type());
   }