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

Source/core/events/PageTransitionEvent.h

 /*
  * Copyright (C) 2009 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 39 matching lines @@
     {
         return adoptRef(new PageTransitionEvent(type, initializer));
     }
 
     virtual ~PageTransitionEvent();
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
     bool persisted() const { return m_persisted; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     PageTransitionEvent();
     PageTransitionEvent(const AtomicString& type, bool persisted);
     PageTransitionEvent(const AtomicString&, const PageTransitionEventInit&);
 
     bool m_persisted;
 };
 
 } // namespace WebCore
 
 #endif // PageTransitionEvent_h