MSE: Implement SourceBuffer.trackDefaults

Source/modules/mediasource/SourceBuffer.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 51 matching lines @@
 
     // Append segment data.
     [RaisesException] void appendBuffer(ArrayBuffer data);
     [RaisesException] void appendBuffer(ArrayBufferView data);
 
     [RaisesException, RuntimeEnabled=Stream] void appendStream(Stream stream, optional unsigned long long maxSize);
 
     // Abort the current segment append sequence.
     [RaisesException] void abort();
     [RaisesException] void remove(double start, unrestricted double end);
+
+    // Gets or sets the TrackDefaultList this SourceBuffer may consult during
+    // the initialization segment algorithm.
+    [RaisesException=Setter, RuntimeEnabled=MediaSourceExperimental] attribute TrackDefaultList trackDefaults;
 };