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

Source/modules/encryptedmedia/MediaKeyNeededEvent.h

 /*
  * Copyright (C) 2012 Google 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 @@
     static PassRefPtr<MediaKeyNeededEvent> create(const AtomicString& type, const MediaKeyNeededEventInit& initializer)
     {
         return adoptRef(new MediaKeyNeededEvent(type, initializer));
     }
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
     String contentType() const { return m_contentType; }
     Uint8Array* initData() const { return m_initData.get(); }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     MediaKeyNeededEvent();
     MediaKeyNeededEvent(const AtomicString& type, const MediaKeyNeededEventInit& initializer);
 
     String m_contentType;
     RefPtr<Uint8Array> m_initData;
 };
 
 } // namespace WebCore
 
 #endif