Implement mapping blink track id to demuxer streams

media/base/demuxer.h

Index: media/base/demuxer.h
diff --git a/media/base/demuxer.h b/media/base/demuxer.h
index 9c6607f9cb212555c5eee02d494b2b2e407d52a5..3f533b33151fea3c7f85a36efafde522a663d784 100644
--- a/media/base/demuxer.h
+++ b/media/base/demuxer.h
@@ -67,7 +67,7 @@ class MEDIA_EXPORT Demuxer : public DemuxerStreamProvider {
                           const std::vector<uint8_t>& init_data)>;
 
   // Notifies demuxer clients that media track configuration has been updated
-  // (e.g. the inital stream metadata has been parsed successfully, or a new
+  // (e.g. the initial stream metadata has been parsed successfully, or a new
   // init segment has been parsed successfully in MSE case).
   using MediaTracksUpdatedCB =
       base::Callback<void(std::unique_ptr<MediaTracks>)>;
@@ -135,6 +135,18 @@ class MEDIA_EXPORT Demuxer : public DemuxerStreamProvider {
   // Returns the memory usage in bytes for the demuxer.
   virtual int64_t GetMemoryUsage() const = 0;
 
+  // Notifies the demuxer that blink track ids have been assigned to |tracks|.

[xhwang, 2016/06/03 18:52:45]

nit: "media" doesn't know about blink. So the |track_id| should be a generic 
concept in "media" as well.

+  // The |track_ids| collection must contain blink track ids in the same
+  // sequence as media tracks in |tracks|. The collection sizes must be the
+  // same.
+  virtual void OnTrackIdsAssigned(const MediaTracks& tracks,
+                                  const std::vector<unsigned>& track_ids) = 0;

[xhwang, 2016/06/03 18:52:45]

We don't like to use "unsigned" or "unsigned int" in chromium. I see that this
is coming from blink, but typically we'll just use uint32_t in chromium:

https://google.github.io/styleguide/cppguide.html#Integer_Types

Also feel free to typedef a media::TrackId if you like.

+
+  // Finds a DemuxerStream corresponding to the given blink |track_id|. Note
+  // that the input track id is blink track id and not bytestream track id.
+  virtual const DemuxerStream* GetDemuxerStreamByTrackId(
+      unsigned track_id) const = 0;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(Demuxer);
 };