Move clearing of dirty descendants bit closer to the last place it's …

…needed.

In the next patch, we'll move logic to identify the children for traversal into
preprocess_children (which will be renamed), and the set_dirty_descendants logic
will move along with it. So left as-is, the code here will clobber the flags.

MozReview-Commit-ID: 7ZskKWD4QC3

components/style/traversal.rs

@@ -666,6 +666,9 @@ where
         } else {
             element.has_dirty_descendants()
         };
+    if context.shared.traversal_flags.for_animation_only() {
+        unsafe { element.unset_animation_only_dirty_descendants(); }
+    }
 
     // Preprocess children, propagating restyle hints and handling sibling
     // relationships.
@@ -695,10 +698,6 @@ where
         data.clear_restyle_state();
     }
 
-    if context.shared.traversal_flags.for_animation_only() {
-        unsafe { element.unset_animation_only_dirty_descendants(); }
-    }
-
     // There are two cases when we want to clear the dity descendants bit here
     // after styling this element.
     //