Use base::Optional for selected video track.

media/base/pipeline.h

Index: media/base/pipeline.h
diff --git a/media/base/pipeline.h b/media/base/pipeline.h
index cf3c69ee136abeeec3b51de73087573575b26686..629658320dbe30a16a8977eb779cffdd37404449 100644
--- a/media/base/pipeline.h
+++ b/media/base/pipeline.h
@@ -82,10 +82,10 @@ class MEDIA_EXPORT Pipeline {
   virtual void OnEnabledAudioTracksChanged(
       const std::vector<MediaTrack::Id>& enabledTrackIds) = 0;
 
-  // |trackId| either empty, which means no video track is selected, or contain
-  // one element - the selected video track id.
+  // |trackId| is either empty, which means no video track is selected, or

[watk, 2017/02/23 20:57:54]

s/trackId/selectedTrackId/

Also why are we using camelcase here? AFAIK the only place the other style needs
to be used is where we're overriding a blink interface.

Looks like there are a lot of instances of this naming style in the selected
track changed code (currTime too) so I won't comment on them all.

[servolk, 2017/02/23 21:15:48]

Done.

+  // contains the selected video track id.
   virtual void OnSelectedVideoTrackChanged(
-      const std::vector<MediaTrack::Id>& selectedTrackId) = 0;
+      base::Optional<MediaTrack::Id> selectedTrackId) = 0;
 
   // Stops the pipeline. This is a blocking function.
   // If the pipeline is started, it must be stopped before destroying it.