Make get() and set() on MutNullableHeap use the correct types.

get() must always return a rooted value, because we have no way of
ensuring the value won't be invalidated. set() takes an &T because it's
convenient; there isn't any need to expose JS<T>.

components/script/dom/uievent.rs

@@ -92,7 +92,7 @@ impl UIEventMethods for UIEvent {
         }
 
         event.InitEvent(type_, can_bubble, cancelable);
-        self.view.set(view.map(JS::from_ref));
+        self.view.set(view);
         self.detail.set(detail);
     }
 }