Oilpan: Make MediaControllers ExecutionContext a weak pointer.

Source/core/html/MediaController.cpp

 /*
  * Copyright (C) 2011 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 595 matching lines @@
         return;
 
     scheduleEvent(EventTypeNames::timeupdate);
     m_previousTimeupdateTime = now;
 }
 
 void MediaController::trace(Visitor* visitor)
 {
     visitor->trace(m_mediaElements);
     visitor->trace(m_pendingEventsQueue);
+    visitor->trace(m_executionContext);
     EventTargetWithInlineData::trace(visitor);
 }
 
 }