Oilpan: Change persistent handles in the Event hierarchy to members

Source/core/events/PopStateEvent.cpp

 /*
  * Copyright (C) 2009 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 63 matching lines @@
     return adoptRef(new PopStateEvent(type, initializer));
 }
 
 const AtomicString& PopStateEvent::interfaceName() const
 {
     return EventNames::PopStateEvent;
 }
 
 void PopStateEvent::trace(Visitor* visitor)
 {
+    visitor->trace(m_history);
     Event::trace(visitor);
 }
 
 } // namespace WebCore