Generate and assign media track ids in demuxers.

media/base/media_tracks.cc

Index: media/base/media_tracks.cc
diff --git a/media/base/media_tracks.cc b/media/base/media_tracks.cc
index 6f3d2c53b6442b6ff0d9d2ee146cfc30a1a039ef..9cfbfbe0bd9fcddc30d0c9ee57d71b717004c3a8 100644
--- a/media/base/media_tracks.cc
+++ b/media/base/media_tracks.cc
@@ -15,30 +15,36 @@ MediaTracks::MediaTracks() {}
 
 MediaTracks::~MediaTracks() {}
 
-void MediaTracks::AddAudioTrack(const AudioDecoderConfig& config,
-                                StreamParser::TrackId bytestream_track_id,
-                                const std::string& kind,
-                                const std::string& label,
-                                const std::string& language) {
+MediaTrack* MediaTracks::AddAudioTrack(
+    const AudioDecoderConfig& config,
+    StreamParser::TrackId bytestream_track_id,
+    const std::string& kind,
+    const std::string& label,
+    const std::string& language) {
   DCHECK(config.IsValidConfig());
   CHECK(audio_configs_.find(bytestream_track_id) == audio_configs_.end());
   std::unique_ptr<MediaTrack> track = base::WrapUnique(new MediaTrack(
       MediaTrack::Audio, bytestream_track_id, kind, label, language));
+  MediaTrack* track_ptr = track.get();
   tracks_.push_back(std::move(track));
   audio_configs_[bytestream_track_id] = config;
+  return track_ptr;
 }
 
-void MediaTracks::AddVideoTrack(const VideoDecoderConfig& config,
-                                StreamParser::TrackId bytestream_track_id,
-                                const std::string& kind,
-                                const std::string& label,
-                                const std::string& language) {
+MediaTrack* MediaTracks::AddVideoTrack(
+    const VideoDecoderConfig& config,
+    StreamParser::TrackId bytestream_track_id,
+    const std::string& kind,
+    const std::string& label,
+    const std::string& language) {
   DCHECK(config.IsValidConfig());
   CHECK(video_configs_.find(bytestream_track_id) == video_configs_.end());
   std::unique_ptr<MediaTrack> track = base::WrapUnique(new MediaTrack(
       MediaTrack::Video, bytestream_track_id, kind, label, language));
+  MediaTrack* track_ptr = track.get();
   tracks_.push_back(std::move(track));
   video_configs_[bytestream_track_id] = config;
+  return track_ptr;
 }
 
 const AudioDecoderConfig& MediaTracks::getAudioConfig(