Use base::Optional for selected video track.

media/base/media_url_demuxer.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_BASE_MEDIA_URL_DEMUXER_H_
 #define MEDIA_BASE_MEDIA_URL_DEMUXER_H_
 
 #include <stddef.h>
 
 #include "base/callback.h"
@@ skipping 41 matching lines @@
   void StartWaitingForSeek(base::TimeDelta seek_time) override;
   void CancelPendingSeek(base::TimeDelta seek_time) override;
   void Seek(base::TimeDelta time, const PipelineStatusCB& status_cb) override;
   void Stop() override;
   void AbortPendingReads() override;
   base::TimeDelta GetStartTime() const override;
   base::Time GetTimelineOffset() const override;
   int64_t GetMemoryUsage() const override;
   void OnEnabledAudioTracksChanged(const std::vector<MediaTrack::Id>& track_ids,
                                    base::TimeDelta currTime) override;
-  void OnSelectedVideoTrackChanged(const std::vector<MediaTrack::Id>& track_ids,
+  void OnSelectedVideoTrackChanged(base::Optional<MediaTrack::Id> track_id,
                                    base::TimeDelta currTime) override;
 
  private:
   MediaUrlParams params_;
 
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaUrlDemuxer);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_MEDIA_URL_DEMUXER_H_