Use base::Optional for selected video track.

media/filters/chunk_demuxer.cc

Index: media/filters/chunk_demuxer.cc
diff --git a/media/filters/chunk_demuxer.cc b/media/filters/chunk_demuxer.cc
index 0f5bd36c37872708d2bc357feb83269f4920e38a..5d24cbf729a80ce848d7ea7b98e8937822fe9012 100644
--- a/media/filters/chunk_demuxer.cc
+++ b/media/filters/chunk_demuxer.cc
@@ -718,14 +718,12 @@ void ChunkDemuxer::OnEnabledAudioTracksChanged(
 }
 
 void ChunkDemuxer::OnSelectedVideoTrackChanged(
-    const std::vector<MediaTrack::Id>& track_ids,
+    base::Optional<MediaTrack::Id> track_id,
     base::TimeDelta currTime) {
-  DCHECK_LE(track_ids.size(), 1u);
-
   base::AutoLock auto_lock(lock_);
   ChunkDemuxerStream* 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());
   }