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..1b97622d966aa41e06dda4cae298823b55198b50 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 +32,29 @@ 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's initialization segment.
+  // But we might have multiple bytestreams (MediaSource might have multiple
+  // SourceBuffers attached to it, which translates into ChunkDemuxer having
+  // multiple MediaSourceStates and multiple bytestreams) or subsequent init
+  // segments may redefine the bytestream ids. 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_;