Add has_attr method to TElement.

If this is all the information the caller needs, we can get it from gecko without
worrying about atomization and string conversions.

components/script/layout_wrapper.rs

@@ -386,6 +386,11 @@ impl<'le> TElement for ServoLayoutElement<'le> {
         self.element.get_state_for_layout()
     }
 
+    #[inline]
+    fn has_attr(&self, namespace: &Namespace, attr: &Atom) -> bool {
+        self.get_attr(namespace, attr).is_some()
+    }
+
     #[inline]
     fn get_attr(&self, namespace: &Namespace, name: &Atom) -> Option<&str> {
         unsafe {