Use base::Optional for selected video track.

media/filters/chunk_demuxer.h

Index: media/filters/chunk_demuxer.h
diff --git a/media/filters/chunk_demuxer.h b/media/filters/chunk_demuxer.h
index f18cae4dd574191d2a861b01fbd7de99bfb99ca8..6537ef0229e152b786413c7bc50d26a30f692fd2 100644
--- a/media/filters/chunk_demuxer.h
+++ b/media/filters/chunk_demuxer.h
@@ -239,10 +239,11 @@ class MEDIA_EXPORT ChunkDemuxer : public Demuxer {
   base::TimeDelta GetHighestPresentationTimestamp(const std::string& id) const;
 
   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;
 
   // Appends media data to the source buffer associated with |id|, applying
   // and possibly updating |*timestamp_offset| during coded frame processing.