Rename Root::root_ref() to Root::r().

As it will be used much more widely after the upcoming changes, this limits
the effort reading and writing the method calls.

components/script/dom/domstringmap.rs

@@ -29,7 +29,7 @@ impl DOMStringMap {
     pub fn new(element: JSRef<HTMLElement>) -> Temporary<DOMStringMap> {
         let window = window_from_node(element).root();
         reflect_dom_object(box DOMStringMap::new_inherited(element),
-                           GlobalRef::Window(window.root_ref()), DOMStringMapBinding::Wrap)
+                           GlobalRef::Window(window.r()), DOMStringMapBinding::Wrap)
     }
 }