Use base::Optional for selected video track.

media/base/pipeline_impl.cc

Index: media/base/pipeline_impl.cc
diff --git a/media/base/pipeline_impl.cc b/media/base/pipeline_impl.cc
index b3ad8f4bf4f03ab4f65c7dc28f858335e0811bdb..ef02fa4e929b95bdacfe151bc8b7e7fa8db49e17 100644
--- a/media/base/pipeline_impl.cc
+++ b/media/base/pipeline_impl.cc
@@ -71,10 +71,10 @@ class PipelineImpl::RendererWrapper : public DemuxerHost,
   void OnEnabledAudioTracksChanged(
       const std::vector<MediaTrack::Id>& enabledTrackIds);
 
-  // |trackId| either empty, which means no video track is selected, or contain
-  // one element - the selected video track id.
+  // |selectedTrackId| is either empty, which means no video track is selected,
+  // or contains the selected video track id.
   void OnSelectedVideoTrackChanged(
-      const std::vector<MediaTrack::Id>& selectedTrackId);
+      base::Optional<MediaTrack::Id> selectedTrackId);
 
  private:
   // Contains state shared between main and media thread.
@@ -569,7 +569,7 @@ void PipelineImpl::OnEnabledAudioTracksChanged(
 }
 
 void PipelineImpl::OnSelectedVideoTrackChanged(
-    const std::vector<MediaTrack::Id>& selectedTrackId) {
+    base::Optional<MediaTrack::Id> selectedTrackId) {
   DCHECK(thread_checker_.CalledOnValidThread());
   media_task_runner_->PostTask(
       FROM_HERE,
@@ -607,7 +607,7 @@ void PipelineImpl::RendererWrapper::OnEnabledAudioTracksChanged(
 }
 
 void PipelineImpl::RendererWrapper::OnSelectedVideoTrackChanged(
-    const std::vector<MediaTrack::Id>& selectedTrackId) {
+    base::Optional<MediaTrack::Id> selectedTrackId) {
   DCHECK(media_task_runner_->BelongsToCurrentThread());
 
   // If the pipeline has been created, but not started yet, we may still receive