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

Source/core/events/KeyboardEvent.cpp

Index: Source/core/events/KeyboardEvent.cpp
diff --git a/Source/core/events/KeyboardEvent.cpp b/Source/core/events/KeyboardEvent.cpp
index 93034ada1130abb4f8c79f821bcb9155c1e0de2b..cbc3cf4839b8fa5d9dbb67291a9c767c2d0f7b87 100644
--- a/Source/core/events/KeyboardEvent.cpp
+++ b/Source/core/events/KeyboardEvent.cpp
@@ -217,6 +217,11 @@ int KeyboardEvent::which() const
     return keyCode();
 }
 
+void KeyboardEvent::trace(Visitor* visitor)
+{
+    UIEventWithKeyState::trace(visitor);
+}
+
 PassRefPtr<KeyboardEventDispatchMediator> KeyboardEventDispatchMediator::create(PassRefPtr<KeyboardEvent> event)
 {
     return adoptRef(new KeyboardEventDispatchMediator(event));