Use base::Optional for selected video track.

media/base/pipeline.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_PIPELINE_H_
 #define MEDIA_BASE_PIPELINE_H_
 
 #include <memory>
 
 #include "base/memory/ref_counted.h"
@@ skipping 60 matching lines @@
 
   // Build a pipeline to using the given |demuxer| and |renderer| to construct
   // a filter chain, executing |seek_cb| when the initial seek has completed.
   // Methods on PipelineClient may be called up until Stop() has completed.
   // It is an error to call this method after the pipeline has already started.
   virtual void Start(Demuxer* demuxer,
                      std::unique_ptr<Renderer> renderer,
                      Client* client,
                      const PipelineStatusCB& seek_cb) = 0;
 
-  // |enabledTrackIds| contains track ids of enabled audio tracks.
+  // |enabled_track_ids| contains track ids of enabled audio tracks.
   virtual void OnEnabledAudioTracksChanged(
-      const std::vector<MediaTrack::Id>& enabledTrackIds) = 0;
+      const std::vector<MediaTrack::Id>& enabled_track_ids) = 0;
 
-  // |trackId| either empty, which means no video track is selected, or contain
-  // one element - the selected video track id.
+  // |selected_track_id| is either empty, which means no video track is
+  // selected, or contains the selected video track id.
   virtual void OnSelectedVideoTrackChanged(
-      const std::vector<MediaTrack::Id>& selectedTrackId) = 0;
+      base::Optional<MediaTrack::Id> selected_track_id) = 0;
 
   // Stops the pipeline. This is a blocking function.
   // If the pipeline is started, it must be stopped before destroying it.
   // It it permissible to call Stop() at any point during the lifetime of the
   // pipeline.
   //
   // Once Stop is called any outstanding completion callbacks
   // for Start/Seek/Suspend/Resume or Client methods will *not* be called.
   virtual void Stop() = 0;
 
@@ skipping 70 matching lines @@
   // Gets the current pipeline statistics.
   virtual PipelineStatistics GetStatistics() const = 0;
 
   virtual void SetCdm(CdmContext* cdm_context,
                       const CdmAttachedCB& cdm_attached_cb) = 0;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_PIPELINE_H_