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

Source/core/events/Event.cpp

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 194 matching lines @@
     size_t eventPathSize = m_eventPath->size();
     for (size_t i = 0; i < eventPathSize; ++i) {
         if (node == (*m_eventPath)[i].node()) {
             ASSERT((*m_eventPath)[i].eventPath());
             return (*m_eventPath)[i].eventPath();
         }
     }
     return StaticNodeList::createEmpty();
 }
 
+void Event::trace(Visitor*)
+{
+}
+
 } // namespace WebCore