Oilpan: Fix untraced weak members.

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 262 matching lines @@
         m_attachedElement->setCloseMediaSourceWhenFinalizing();
         m_attachedElement.clear();
     }
 #endif
 }
 
 DEFINE_TRACE(MediaSource)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_asyncEventQueue);
+    visitor->trace(m_attachedElement);

[sof, 2015/03/03 08:50:36]

the weak callback handles this weak member; we shouldn't separately trace it
(and register another weak callback.)

 #endif
     visitor->trace(m_sourceBuffers);
     visitor->trace(m_activeSourceBuffers);
     visitor->template registerWeakMembers<MediaSource, &MediaSource::clearWeakMembers>(this);
     RefCountedGarbageCollectedEventTargetWithInlineData<MediaSource>::trace(visitor);
     ActiveDOMObject::trace(visitor);
 }
 
 void MediaSource::setWebMediaSourceAndOpen(PassOwnPtr<WebMediaSource> webMediaSource)
 {
@@ skipping 320 matching lines @@
 
     m_asyncEventQueue->enqueueEvent(event.release());
 }
 
 URLRegistry& MediaSource::registry() const
 {
     return MediaSourceRegistry::registry();
 }
 
 } // namespace blink