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/attr.rs

@@ -315,7 +315,7 @@ impl Attr {
             }
             (old, new) => assert!(old == new)
         }
-        self.owner.set(owner.map(JS::from_ref))
+        self.owner.set(owner);
     }
 
     pub fn owner(&self) -> Option<Root<Element>> {