Generate and assign media track ids in demuxers.

media/base/demuxer.h

Index: media/base/demuxer.h
diff --git a/media/base/demuxer.h b/media/base/demuxer.h
index 9c6607f9cb212555c5eee02d494b2b2e407d52a5..b113da60edf8d7652d2740d23a4551ff01c7c5f1 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 +135,9 @@ class MEDIA_EXPORT Demuxer : public DemuxerStreamProvider {
   // Returns the memory usage in bytes for the demuxer.
   virtual int64_t GetMemoryUsage() const = 0;
 
+  virtual const DemuxerStream* GetDemuxerStreamByTrackId(

[xhwang, 2016/06/09 06:06:02]

This is actually not used in this CL. Also, in your other CL, it's still TBD
whether we want/need to do this. Shall we drop this change from this CL and then
iterate on this later?

[servolk, 2016/06/09 19:14:46]

At the moment I'm inclined to think it's better to keep this API, since it's
more generic and useful. I'm not sure if I fully understand your suggestions in
https://codereview.chromium.org/1935873002/#msg19. I think this is a useful
generic API that essentially allows translating media track ids (which is what
blink operates with) into demuxer streams, and the functionality might be useful
beyond just enabling/disabling tracks. Let's have some further discussion in
https://codereview.chromium.org/1935873002 before removing this. I'll add my
thoughts in there shortly.

[xhwang, 2016/06/09 21:04:16]

In general we don't add an API because it's generic and could be useful. We add
an API because we use it.

I think it'd be great if we can hide the track ID as an implementation detail of
the demuxer stack and the rest of the media pipeline don't need to know about
it. This API definitely opens a door for that.

+      std::string track_id) const = 0;

[xhwang, 2016/06/09 06:06:02]

const ref

[servolk, 2016/06/09 19:14:46]

Done.

+
  private:
   DISALLOW_COPY_AND_ASSIGN(Demuxer);
 };