Usage of JSRef<Attr> in before_remove_attr & after_set_attr

JSRef<Attr> does not require allocating a DOMString for value, which are
unused in most cases. It also provides more access to Attr data.
auto merge of #3757 : brunoabinader/servo/content_changed, r=jdm

```JSRef<Attr>``` does not require allocating a ```DOMString``` for value, which are unused in most cases. It also provides more access to ```Attr``` data.