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

Source/core/events/MouseEvent.cpp

Index: Source/core/events/MouseEvent.cpp
diff --git a/Source/core/events/MouseEvent.cpp b/Source/core/events/MouseEvent.cpp
index 0aefd141b2ce3a826ca611aa9b63779f1080a516..9788317e47976a1644172f110cf7a0e06070ab9b 100644
--- a/Source/core/events/MouseEvent.cpp
+++ b/Source/core/events/MouseEvent.cpp
@@ -190,6 +190,12 @@ Node* MouseEvent::fromElement() const
     return target() ? target()->toNode() : 0;
 }
 
+void MouseEvent::trace(Visitor* visitor)
+{
+    visitor->trace(m_clipboard);
+    MouseRelatedEvent::trace(visitor);
+}
+
 PassRefPtr<SimulatedMouseEvent> SimulatedMouseEvent::create(const AtomicString& eventType, PassRefPtr<AbstractView> view, PassRefPtr<Event> underlyingEvent)
 {
     return adoptRef(new SimulatedMouseEvent(eventType, view, underlyingEvent));
@@ -219,6 +225,11 @@ SimulatedMouseEvent::SimulatedMouseEvent(const AtomicString& eventType, PassRefP
     }
 }
 
+void SimulatedMouseEvent::trace(Visitor* visitor)
+{
+    MouseEvent::trace(visitor);
+}
+
 PassRefPtr<MouseEventDispatchMediator> MouseEventDispatchMediator::create(PassRefPtr<MouseEvent> mouseEvent, MouseEventType mouseEventType)
 {
     return adoptRef(new MouseEventDispatchMediator(mouseEvent, mouseEventType));