Change get_attr() to get_attr_val_for_layout().

The old code was used by both layout and script, but was erroneously
borrowing for the layout case (which causes parallelism
problems). script now uses only `value_ref()` or `get_attribute()`, and
layout now has its own unsafe version that dances around the borrows of
`@mut Attr`.

src/components/script/dom/attr.rs

@@ -65,6 +65,10 @@ impl Attr {
         util::swap(&mut self.value, &mut value);
         value
     }
+
+    pub fn value_ref<'a>(&'a self) -> &'a str {
+        self.value.as_slice()
+    }
 }
 
 impl Attr {