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

Source/core/events/HashChangeEvent.h

 /*
  * Copyright (C) 2010 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.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 50 matching lines @@
 
         m_oldURL = oldURL;
         m_newURL = newURL;
     }
 
     const String& oldURL() const { return m_oldURL; }
     const String& newURL() const { return m_newURL; }
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::HashChangeEvent; }
 
+    virtual void trace(Visitor* visitor) OVERRIDE { Event::trace(visitor); }
+
 private:
     HashChangeEvent()
     {
         ScriptWrappable::init(this);
     }
 
     HashChangeEvent(const String& oldURL, const String& newURL)
         : Event(EventTypeNames::hashchange, false, false)
         , m_oldURL(oldURL)
         , m_newURL(newURL)
     {
         ScriptWrappable::init(this);
     }
 
     HashChangeEvent(const AtomicString& type, const HashChangeEventInit& initializer)
         : Event(type, initializer)
         , m_oldURL(initializer.oldURL)
         , m_newURL(initializer.newURL)
     {
         ScriptWrappable::init(this);
     }
 
     String m_oldURL;
     String m_newURL;
 };
 
 } // namespace WebCore
 
 #endif // HashChangeEvent_h