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

Source/modules/serviceworkers/InstallPhaseEvent.cpp

 /*
  * Copyright (C) 2014 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 42 matching lines @@
 InstallPhaseEvent::InstallPhaseEvent()
 {
 }
 
 InstallPhaseEvent::InstallPhaseEvent(const AtomicString& type, const EventInit& initializer, PassRefPtr<WaitUntilObserver> observer)
     : Event(type, initializer)
     , m_observer(observer)
 {
 }
 
+void InstallPhaseEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore