Auto merge of #8060 - nox:deref-js, r=Ms2ger

Implement Deref<Target=T> for JS<T> where T: Reflectable

We can only borrow `JS<T>` from rooted things, so it's safe to deref it.
The only types that provide mutable `JS<T>` things are `MutHeap<JS<T>>` and
`MutNullableHeap<JS<T>>`, which don't actually expose that they contain
`JS<T>` values.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8060)
<!-- Reviewable:end -->

components/script/dom/element.rs

@@ -1176,7 +1176,7 @@ impl ElementMethods for Element {
                 node.owner_doc()
             };
             let window = doc.r().window();
-            NamedNodeMap::new(window.r(), self)
+            NamedNodeMap::new(window, self)
         })
     }