Oilpan: Prepare to move EventDispatchMediator to Oilpan heap.

Source/core/events/MouseEvent.h

Index: Source/core/events/MouseEvent.h
diff --git a/Source/core/events/MouseEvent.h b/Source/core/events/MouseEvent.h
index ff4d69967668a9b869b1756d1a28128f4559b425..fc364c8e8c74d8a77249ea4e662a5d32f4c3eead 100644
--- a/Source/core/events/MouseEvent.h
+++ b/Source/core/events/MouseEvent.h
@@ -125,7 +125,7 @@ private:
 class MouseEventDispatchMediator FINAL : public EventDispatchMediator {
 public:
     enum MouseEventType { SyntheticMouseEvent, NonSyntheticMouseEvent};
-    static PassRefPtr<MouseEventDispatchMediator> create(PassRefPtrWillBeRawPtr<MouseEvent>, MouseEventType = NonSyntheticMouseEvent);
+    static PassRefPtrWillBeRawPtr<MouseEventDispatchMediator> create(PassRefPtrWillBeRawPtr<MouseEvent>, MouseEventType = NonSyntheticMouseEvent);
 
 private:
     explicit MouseEventDispatchMediator(PassRefPtrWillBeRawPtr<MouseEvent>, MouseEventType);