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 b5f1cdb5a9348e70a97dba6f14bc232cc237b173..3ab13336da364ad14f54c2f15609bf8ed2c659aa 100644
--- a/media/base/media_tracks.h
+++ b/media/base/media_tracks.h
@@ -26,18 +26,20 @@ class MEDIA_EXPORT MediaTracks {
   MediaTracks();
   ~MediaTracks();
 
-  // Callers need to ensure that track id is unique.
-  void AddAudioTrack(const AudioDecoderConfig& config,
-                     StreamParser::TrackId byteStreamTrackId,
-                     const std::string& kind,
-                     const std::string& label,
-                     const std::string& language);
-  // Callers need to ensure that track id is unique.
-  void AddVideoTrack(const VideoDecoderConfig& config,
-                     StreamParser::TrackId byteStreamTrackId,
-                     const std::string& kind,
-                     const std::string& label,
-                     const std::string& language);
+  // Adds a new audio track. The |bytestreamTrackId| must uniquely identify the
+  // track within the bytestream.
+  MediaTrack* AddAudioTrack(const AudioDecoderConfig& config,
+                            StreamParser::TrackId byteStreamTrackId,
+                            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.
+  MediaTrack* AddVideoTrack(const VideoDecoderConfig& config,
+                            StreamParser::TrackId byteStreamTrackId,
+                            const std::string& kind,
+                            const std::string& label,
+                            const std::string& language);
 
   const MediaTracksCollection& tracks() const { return tracks_; }