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

Source/core/html/MediaKeyEvent.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 50 matching lines @@
 
     String keySystem() const { return m_keySystem; }
     String sessionId() const { return m_sessionId; }
     Uint8Array* initData() const { return m_initData.get(); }
     Uint8Array* message() const { return m_message.get(); }
     String defaultURL() const { return m_defaultURL; }
     MediaKeyError* errorCode() const { return m_errorCode.get(); }
     MediaKeyError* errorCode(bool& isNull) const { isNull = !m_errorCode; return m_errorCode.get(); }
     unsigned short systemCode() const { return m_systemCode; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     MediaKeyEvent();
     MediaKeyEvent(const AtomicString& type, const MediaKeyEventInit& initializer);
 
     String m_keySystem;
     String m_sessionId;
     RefPtr<Uint8Array> m_initData;
     RefPtr<Uint8Array> m_message;
     String m_defaultURL;
     RefPtr<MediaKeyError> m_errorCode;
     unsigned short m_systemCode;
 };
 
 } // namespace WebCore
 
 #endif