Oilpan: Allocate all EventTarget derived types on the manged heap.

Source/core/dom/MessagePort.h

Index: Source/core/dom/MessagePort.h
diff --git a/Source/core/dom/MessagePort.h b/Source/core/dom/MessagePort.h
index d51adf098e3d9603441f181f34c2a742f5942a53..cbec8fdb11c4a39d7020c0baa574e24296e8fa28 100644
--- a/Source/core/dom/MessagePort.h
+++ b/Source/core/dom/MessagePort.h
@@ -56,12 +56,13 @@ typedef Vector<RefPtr<MessagePort>, 1> MessagePortArray;
 // Not to be confused with blink::WebMessagePortChannelArray; this one uses Vector and OwnPtr instead of WebVector and raw pointers.
 typedef Vector<OwnPtr<blink::WebMessagePortChannel>, 1> MessagePortChannelArray;
 
-class MessagePort FINAL : public RefCounted<MessagePort>
+class MessagePort FINAL : public RefCountedWillBeRefCountedGarbageCollected<MessagePort>
     , public ActiveDOMObject
     , public EventTargetWithInlineData
     , public ScriptWrappable
     , public blink::WebMessagePortChannelClient {
     REFCOUNTED_EVENT_TARGET(MessagePort);
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(MessagePort);
 public:
     static PassRefPtr<MessagePort> create(ExecutionContext&);
     virtual ~MessagePort();