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 87e4bc4f6dd9e65cc9f4f9ae985b1414452d299a..d65fc1ff3a0b1ff08999addc3a34dcd29d05db7b 100644
--- a/Source/core/events/BeforeUnloadEvent.h
+++ b/Source/core/events/BeforeUnloadEvent.h
@@ -33,9 +33,9 @@ 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;