Add has_current_styles_for_traversal().

In animation-only restyle, we just need to check the element
has animation restyle hints or has recascade self which is a result
of animation-only restyle for ancestors.
has_current_styles() in Servo_ResolveStyle() is intentionally left there,
it will be changed in a subsequent patch.

components/style/sharing/mod.rs

@@ -675,7 +675,7 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
         }
 
         let data = candidate.element.borrow_data().unwrap();
-        debug_assert!(target.has_current_styles(&data));
+        debug_assert!(target.has_current_styles_for_traversal(&data, shared.traversal_flags));
 
         debug!("Sharing style between {:?} and {:?}",
                target.element, candidate.element);