Make Event RefCountedGarbageCollected and implement trace() methods to the Event hierarcy

Source/core/events/BeforeUnloadEvent.h

 /*
  * 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, 2004, 2005, 2006 Apple Computer, Inc.
  * Copyright (C) 2013 Samsung Electronics.
  *
  * 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
@@ skipping 28 matching lines @@
         return adoptRef(new BeforeUnloadEvent);
     }
 
     virtual bool isBeforeUnloadEvent() const OVERRIDE;
 
     void setReturnValue(const String& returnValue) { m_returnValue = returnValue; }
     String returnValue() const { return m_returnValue; }
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::BeforeUnloadEvent; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     BeforeUnloadEvent();
 
     String m_returnValue;
 };
 
 DEFINE_EVENT_TYPE_CASTS(BeforeUnloadEvent);
 
 } // namespace WebCore
 
 #endif // BeforeUnloadEvent_h