Use base::Optional for selected video track.

media/base/demuxer.h

 // Copyright (c) 2012 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_DEMUXER_H_
 #define MEDIA_BASE_DEMUXER_H_
 
 #include <stdint.h>
 
 #include <memory>
 #include <vector>
 
 #include "base/macros.h"
+#include "base/optional.h"
 #include "base/time/time.h"
 #include "media/base/data_source.h"
 #include "media/base/demuxer_stream.h"
 #include "media/base/eme_constants.h"
 #include "media/base/media_export.h"
 #include "media/base/media_resource.h"
 #include "media/base/media_track.h"
 #include "media/base/pipeline_status.h"
 #include "media/base/ranges.h"
 
@@ skipping 112 matching lines @@
   // Returns Time represented by presentation timestamp 0.
   // If the timstamps are not associated with a Time, then
   // a null Time is returned.
   virtual base::Time GetTimelineOffset() const = 0;
 
   // Returns the memory usage in bytes for the demuxer.
   virtual int64_t GetMemoryUsage() const = 0;
 
   virtual void OnEnabledAudioTracksChanged(
       const std::vector<MediaTrack::Id>& track_ids,
-      base::TimeDelta currTime) = 0;
+      base::TimeDelta curr_time) = 0;
 
-  // |track_ids| is either empty or contains a single video track id.
+  // |track_id| either contains the selected video track id or is null,
+  // indicating that all video tracks are deselected/disabled.
   virtual void OnSelectedVideoTrackChanged(
-      const std::vector<MediaTrack::Id>& track_ids,
-      base::TimeDelta currTime) = 0;
+      base::Optional<MediaTrack::Id> track_id,
+      base::TimeDelta curr_time) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Demuxer);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_DEMUXER_H_