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

Source/core/xml/XMLHttpRequestProgressEvent.h

 /*
  * Copyright (C) 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Julien Chaffraix <jchaffraix@webkit.org>. 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
@@ skipping 31 matching lines @@
     {
         return adoptRef(new XMLHttpRequestProgressEvent(type, lengthComputable, loaded, total));
     }
 
     // Those 2 synonyms are included for compatibility with Firefox.
     unsigned long long position() const { return loaded(); }
     unsigned long long totalSize() const { return total(); }
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::XMLHttpRequestProgressEvent; }
 
+    virtual void trace(Visitor* visitor) OVERRIDE { ProgressEvent::trace(visitor); }
+
 private:
     XMLHttpRequestProgressEvent()
     {
         ScriptWrappable::init(this);
     }
 
     XMLHttpRequestProgressEvent(const AtomicString& type, bool lengthComputable, unsigned long long loaded, unsigned long long total)
         : ProgressEvent(type, lengthComputable, loaded, total)
     {
         ScriptWrappable::init(this);
     }
 };
 
 } // namespace WebCore
 
 #endif // XMLHttpRequestProgressEvent_h