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

Source/modules/webmidi/MIDIMessageEvent.h

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 50 matching lines @@
     static PassRefPtr<MIDIMessageEvent> create(const AtomicString& type, const MIDIMessageEventInit& initializer)
     {
         return adoptRef(new MIDIMessageEvent(type, initializer));
     }
 
     double receivedTime() { return m_receivedTime; }
     PassRefPtr<Uint8Array> data() { return m_data; }
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::MIDIMessageEvent; }
 
+    virtual void trace(Visitor* visitor) OVERRIDE { Event::trace(visitor); }
+
 private:
     MIDIMessageEvent()
         : m_receivedTime(0)
     {
         ScriptWrappable::init(this);
     }
 
     MIDIMessageEvent(double receivedTime, PassRefPtr<Uint8Array> data)
         : Event(EventTypeNames::midimessage, true, false)
         , m_receivedTime(receivedTime)
@@ skipping 10 matching lines @@
         ScriptWrappable::init(this);
     }
 
     double m_receivedTime;
     RefPtr<Uint8Array> m_data;
 };
 
 } // namespace WebCore
 
 #endif // MIDIMessageEvent_h