Generate and assign media track ids in demuxers.

media/base/media_track.h

Index: media/base/media_track.h
diff --git a/media/base/media_track.h b/media/base/media_track.h
index 3cd41d736ffa05cd702b6402e5ad91a57b300ada..df1165c957a722136551c9d1ce887fc09e01224d 100644
--- a/media/base/media_track.h
+++ b/media/base/media_track.h
@@ -15,6 +15,7 @@ namespace media {
 class MEDIA_EXPORT MediaTrack {
  public:
   enum Type { Text, Audio, Video };
+  using Id = std::string;
   MediaTrack(Type type,
              StreamParser::TrackId bytestream_track_id,
              const std::string& kind,
@@ -31,9 +32,28 @@ class MEDIA_EXPORT MediaTrack {
   const std::string& label() const { return label_; }
   const std::string& language() const { return language_; }
 
+  Id id() const { return id_; }
+  void set_id(Id id) {
+    DCHECK(id_.empty());
+    DCHECK(!id.empty());
+    id_ = id;
+  }
+
  private:
   Type type_;
+
+  // |bytestream_track_id_| is read from the bytestream and is guaranteed to be
+  // unique only within the scope of single bytestream. But we might have

[wolenetz, 2016/06/13 19:51:37]

nit: s//single bytestream/single bytestream's initialization segment/

Note the following scenario is allowed by MSE:
Single SourceBuffer, containing muxed single audio + single video track.
Init Segment #1: Audio bs_id="1", Video bs_id="2"
Init Segment #2: Audio bs_id="2", Video bs_id="1"

Note that the bytestream ID's were not mapped to the same track.

[servolk, 2016/06/13 22:00:15]

Yep, good catch, thanks, I've updated the comment to mention this.

[wolenetz, 2016/06/13 23:23:57]

Acknowledged.

+  // multiple bytestreams (MediaSource might have multiple SourceBuffers
+  // attached to it, which translates into ChunkDemuxer having multiple
+  // MediaSourceStates and multiple bytestreams). Thus bytestream track ids
+  // are not guaranteed to be unique at the Demuxer and HTMLMediaElement level.
+  // So we generate truly unique media track |id_| on the Demuxer level.
   StreamParser::TrackId bytestream_track_id_;
+  Id id_;
+
+  // These properties are read from input streams by stream parsers as specified
+  // in https://dev.w3.org/html5/html-sourcing-inband-tracks/.
   std::string kind_;
   std::string label_;
   std::string language_;