Use base::Optional for selected video track.

media/base/demuxer.h

Index: media/base/demuxer.h
diff --git a/media/base/demuxer.h b/media/base/demuxer.h
index 71cdc6d14a91518950aa7d1dc943568937dff761..bae29dc3fb967e90822130cc3630e2de7b6118da 100644
--- a/media/base/demuxer.h
+++ b/media/base/demuxer.h
@@ -11,6 +11,7 @@
 #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"
@@ -145,9 +146,10 @@ class MEDIA_EXPORT Demuxer : public MediaResource {
       const std::vector<MediaTrack::Id>& track_ids,
       base::TimeDelta currTime) = 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::Optional<MediaTrack::Id> track_id,
       base::TimeDelta currTime) = 0;
 
  private: