Add [WillBeGarbageCollected] to Event.idl

Source/core/events/BeforeUnloadEvent.h

Index: Source/core/events/BeforeUnloadEvent.h
diff --git a/Source/core/events/BeforeUnloadEvent.h b/Source/core/events/BeforeUnloadEvent.h
index 460ea13930576c2c275e17bb05d15d882b7d4dae..2fe18c03f9baaf93438f5b74ff65bd9626709ceb 100644
--- a/Source/core/events/BeforeUnloadEvent.h
+++ b/Source/core/events/BeforeUnloadEvent.h
@@ -34,9 +34,9 @@ class BeforeUnloadEvent FINAL : public Event {
 public:
     virtual ~BeforeUnloadEvent();
 
-    static PassRefPtr<BeforeUnloadEvent> create()
+    static PassRefPtrWillBeRawPtr<BeforeUnloadEvent> create()
     {
-        return adoptRef(new BeforeUnloadEvent);
+        return adoptRefWillBeRefCountedGarbageCollected(new BeforeUnloadEvent);
     }
 
     virtual bool isBeforeUnloadEvent() const OVERRIDE;