Generate and assign media track ids in demuxers.

media/filters/ffmpeg_demuxer.h

Index: media/filters/ffmpeg_demuxer.h
diff --git a/media/filters/ffmpeg_demuxer.h b/media/filters/ffmpeg_demuxer.h
index def9d2a45d987fd8b9613eff6c895dc25322e043..155c5027ca31f84a24d6c1ed44e8f73c7c8a1cd8 100644
--- a/media/filters/ffmpeg_demuxer.h
+++ b/media/filters/ffmpeg_demuxer.h
@@ -38,6 +38,7 @@
 #include "media/base/decoder_buffer.h"
 #include "media/base/decoder_buffer_queue.h"
 #include "media/base/demuxer.h"
+#include "media/base/media_tracks.h"
 #include "media/base/pipeline_status.h"
 #include "media/base/text_track_config.h"
 #include "media/base/video_decoder_config.h"
@@ -213,6 +214,9 @@ class MEDIA_EXPORT FFmpegDemuxer : public Demuxer {
   void OnEncryptedMediaInitData(EmeInitDataType init_data_type,
                                 const std::string& encryption_key_id);
 
+  const DemuxerStream* GetDemuxerStreamByTrackId(
+      MediaTrack::TrackId track_id) const override;
+
   // Allow FFmpegDemuxerStream to notify us when there is updated information
   // about capacity and what buffered data is available.
   void NotifyCapacityAvailable();
@@ -334,6 +338,9 @@ class MEDIA_EXPORT FFmpegDemuxer : public Demuxer {
 
   const MediaTracksUpdatedCB media_tracks_updated_cb_;
 
+  std::map<MediaTrack::TrackId, const DemuxerStream*>
+      track_id_to_demux_stream_map_;
+
   // NOTE: Weak pointers must be invalidated before all other member variables.
   base::WeakPtrFactory<FFmpegDemuxer> weak_factory_;