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/mouseevent.rs

@@ -205,6 +205,6 @@ impl MouseEventMethods for MouseEvent {
         self.shift_key.set(shiftKeyArg);
         self.meta_key.set(metaKeyArg);
         self.button.set(buttonArg);
-        self.related_target.set(relatedTargetArg.map(JS::from_ref));
+        self.related_target.set(relatedTargetArg);
     }
 }