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

Source/core/events/MutationEvent.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, 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 44 matching lines @@
                                const String& attrName, unsigned short attrChange);
 
         Node* relatedNode() const { return m_relatedNode.get(); }
         String prevValue() const { return m_prevValue; }
         String newValue() const { return m_newValue; }
         String attrName() const { return m_attrName; }
         unsigned short attrChange() const { return m_attrChange; }
 
         virtual const AtomicString& interfaceName() const OVERRIDE;
 
+        virtual void trace(Visitor*) OVERRIDE;
+
     private:
         MutationEvent();
         MutationEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<Node> relatedNode,
                       const String& prevValue, const String& newValue,
                       const String& attrName, unsigned short attrChange);
 
         RefPtr<Node> m_relatedNode;
         String m_prevValue;
         String m_newValue;
         String m_attrName;
         unsigned short m_attrChange;
     };
 
 } // namespace WebCore
 
 #endif // MutationEvent_h