Generate and assign media track ids in demuxers.

media/base/media_tracks.h

Index: media/base/media_tracks.h
diff --git a/media/base/media_tracks.h b/media/base/media_tracks.h
index 068139899df353698d31b95b9e05edb8108c353f..2d7431bcd9efed4b4e169d8d4bcd45c722635fc4 100644
--- a/media/base/media_tracks.h
+++ b/media/base/media_tracks.h
@@ -28,18 +28,18 @@ class MEDIA_EXPORT MediaTracks {
 
   // Adds a new audio track. The |bytestreamTrackId| must uniquely identify the
   // track within the bytestream.
-  void AddAudioTrack(const AudioDecoderConfig& config,
-                     StreamParser::TrackId bytestream_track_id,
-                     const std::string& kind,
-                     const std::string& label,
-                     const std::string& language);
+  MediaTrack* AddAudioTrack(const AudioDecoderConfig& config,
+                            StreamParser::TrackId bytestream_track_id,
+                            const std::string& kind,
+                            const std::string& label,
+                            const std::string& language);
   // Adds a new video track. The |bytestreamTrackId| must uniquely identify the
   // track within the bytestream.
-  void AddVideoTrack(const VideoDecoderConfig& config,
-                     StreamParser::TrackId bytestream_track_id,
-                     const std::string& kind,
-                     const std::string& label,
-                     const std::string& language);
+  MediaTrack* AddVideoTrack(const VideoDecoderConfig& config,
+                            StreamParser::TrackId bytestream_track_id,
+                            const std::string& kind,
+                            const std::string& label,
+                            const std::string& language);
 
   const MediaTracksCollection& tracks() const { return tracks_; }