Don't expose any AtomicRefCell directly from style traits

This lets us experiment with how we store this data on the DOM side.

components/style/traversal.rs

@@ -845,7 +845,7 @@ where
                 //
                 // By consequence, any element without data has no descendants with
                 // data.
-                if kid.get_data().is_some() {
+                if kid.has_data() {
                     kid.clear_data();
                     parents.push(kid);
                 }