Fix MSE GC, make it less aggressive, more spec-compliant (Blink CL)

Source/modules/mediasource/MediaSource.cpp

 /*
  * 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 512 matching lines @@
 
     size_t insertPosition = 0;
     while (insertPosition < m_activeSourceBuffers->length()
         && m_sourceBuffers->find(m_activeSourceBuffers->item(insertPosition)) < indexInSourceBuffers) {
         ++insertPosition;
     }
 
     m_activeSourceBuffers->insert(insertPosition, sourceBuffer);
 }
 
+HTMLMediaElement* MediaSource::getHTMLMediaElement() const
+{
+    return m_attachedElement;
+}
+
 bool MediaSource::isClosed() const
 {
     return readyState() == closedKeyword();
 }
 
 void MediaSource::close()
 {
     setReadyState(closedKeyword());
 }
 
@@ skipping 70 matching lines @@
 
     m_asyncEventQueue->enqueueEvent(event.release());
 }
 
 URLRegistry& MediaSource::registry() const
 {
     return MediaSourceRegistry::registry();
 }
 
 } // namespace blink