Don't traverse any elements that needed only for animation-only resty…

…les in normal traversal.

Before this patch, we were setting the dirty descendants bit in animation-only
restyles and it triggered unnecessary traversal for elements that does not need
the traversal (i.e no need selector matching).

components/style/traversal.rs

@@ -462,7 +462,11 @@ pub trait DomTraversal<E: TElement> : Sync {
                     let el = kid.as_element();
                     if el.as_ref().and_then(|el| el.borrow_data())
                                   .map_or(false, |d| d.has_styles()) {
-                        unsafe { parent.set_dirty_descendants(); }
+                        if self.shared_context().traversal_flags.for_animation_only() {
+                            unsafe { parent.set_animation_only_dirty_descendants(); }
+                        } else {
+                            unsafe { parent.set_dirty_descendants(); }
+                        }
                     }
                 }
                 f(thread_local, kid);

ports/geckolib/glue.rs

@@ -300,7 +300,9 @@ pub extern "C" fn Servo_TraverseSubtree(root: RawGeckoElementBorrowed,
         return false;
     }
 
-    element.has_dirty_descendants() || element.borrow_data().unwrap().restyle.contains_restyle_data()
+    element.has_dirty_descendants() ||
+    element.has_animation_only_dirty_descendants() ||
+    element.borrow_data().unwrap().restyle.contains_restyle_data()
 }
 
 /// Checks whether the rule tree has crossed its threshold for unused nodes, and
@@ -2801,7 +2803,9 @@ pub extern "C" fn Servo_AssertTreeIsClean(root: RawGeckoElementBorrowed) {
 
     let root = GeckoElement(root);
     fn assert_subtree_is_clean<'le>(el: GeckoElement<'le>) {
-        debug_assert!(!el.has_dirty_descendants() && !el.has_animation_only_dirty_descendants());
+        debug_assert!(!el.has_dirty_descendants() && !el.has_animation_only_dirty_descendants(),
+                      "{:?} has still dirty bit {:?} or animation-only dirty bit {:?}",
+                      el, el.has_dirty_descendants(), el.has_animation_only_dirty_descendants());
         for child in el.as_node().traversal_children() {
             if let Some(child) = child.as_element() {
                 assert_subtree_is_clean(child);