Use base::Optional for selected video track.

media/base/pipeline_impl.h

Index: media/base/pipeline_impl.h
diff --git a/media/base/pipeline_impl.h b/media/base/pipeline_impl.h
index b5f5699e9f80bbd86ddf36a3949b829f48f12cdb..a166dd4cbd305d8d2183f495504a9c62520454a2 100644
--- a/media/base/pipeline_impl.h
+++ b/media/base/pipeline_impl.h
@@ -99,10 +99,10 @@ class MEDIA_EXPORT PipelineImpl : public Pipeline {
   void OnEnabledAudioTracksChanged(
       const std::vector<MediaTrack::Id>& enabledTrackIds) override;
 
-  // |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) override;
+      base::Optional<MediaTrack::Id> selectedTrackId) override;
 
  private:
   friend class MediaLog;