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.

components/script/dom/bindings/js.rs

@@ -96,6 +96,16 @@ impl<T: Reflectable> JS<T> {
     }
 }
 
+impl<T: Reflectable> Deref for JS<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // We can only have &JS<T> from a rooted thing, so it's safe to deref
+        // it to &T.
+        unsafe { &**self.ptr }
+    }
+}
+
 impl<T: Reflectable> JSTraceable for JS<T> {
     fn trace(&self, trc: *mut JSTracer) {
         trace_reflector(trc, "", unsafe { (**self.ptr).reflector() });