Oilpan: Prepare to move EventDispatchMediator to Oilpan heap.

Source/core/events/WheelEvent.h

Index: Source/core/events/WheelEvent.h
diff --git a/Source/core/events/WheelEvent.h b/Source/core/events/WheelEvent.h
index edd9158ab91ff277a175efa7a062d36951009214..efe1c7572b2d3c3916172738d3bdf4202d7b6de3 100644
--- a/Source/core/events/WheelEvent.h
+++ b/Source/core/events/WheelEvent.h
@@ -118,7 +118,7 @@ DEFINE_EVENT_TYPE_CASTS(WheelEvent);
 
 class WheelEventDispatchMediator FINAL : public EventDispatchMediator {
 public:
-    static PassRefPtr<WheelEventDispatchMediator> create(const PlatformWheelEvent&, PassRefPtrWillBeRawPtr<AbstractView>);
+    static PassRefPtrWillBeRawPtr<WheelEventDispatchMediator> create(const PlatformWheelEvent&, PassRefPtrWillBeRawPtr<AbstractView>);
 private:
     WheelEventDispatchMediator(const PlatformWheelEvent&, PassRefPtrWillBeRawPtr<AbstractView>);
     WheelEvent* event() const;