Oilpan: Reduce RefPtr to WorkerGlobalScope.

Source/core/events/Event.cpp

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 217 matching lines @@
     for (size_t i = 0; i < eventPathSize; ++i) {
         if (node == (*m_eventPath)[i].node()) {
             return (*m_eventPath)[i].treeScopeEventContext().ensureEventPath(*m_eventPath);
         }
     }
     return StaticNodeList::createEmpty();
 }
 
 void Event::trace(Visitor* visitor)
 {
+    visitor->trace(m_target);
     visitor->trace(m_underlyingEvent);
     visitor->trace(m_eventPath);
 }
 
 } // namespace WebCore