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

Source/core/events/MutationEvent.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 53 matching lines @@
     m_newValue = newValue;
     m_attrName = attrName;
     m_attrChange = attrChange;
 }
 
 const AtomicString& MutationEvent::interfaceName() const
 {
     return EventNames::MutationEvent;
 }
 
+void MutationEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore