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

Source/core/events/OverflowEvent.cpp

 /*
  * Copyright (C) 2006, 2007 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 58 matching lines @@
     , m_verticalOverflow(initializer.verticalOverflow)
 {
     ScriptWrappable::init(this);
 }
 
 const AtomicString& OverflowEvent::interfaceName() const
 {
     return EventNames::OverflowEvent;
 }
 
+void OverflowEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
 }
+
+}