Generate and assign media track ids in demuxers.

media/base/media_track.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_BASE_MEDIA_TRACK_H_
 #define MEDIA_BASE_MEDIA_TRACK_H_
 
 #include <string>
 
 #include "media/base/media_export.h"
 #include "media/base/stream_parser.h"
 
 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,
              const std::string& label,
              const std::string& lang);
   ~MediaTrack();
 
   Type type() const { return type_; }
 
   StreamParser::TrackId bytestream_track_id() const {
     return bytestream_track_id_;
   }
   const std::string& kind() const { return kind_; }
   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_;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_MEDIA_TRACK_H_