Hoist StyleNew{Children,Subtree} into their own paths.

The buggy animation handling isn't a regression, since currently we pass
UnstyledChildrenOnly in those cases, which blocks the animation traversal
in Servo_TraverseSubtree.

In general I really wanted to handle these two paths together. But there's
enough broken with the NewChildren path that I wanted to scope the buginess
as tightly as possible. And I really need to separate the handling here from
StyleDocument() to make the restyle root stuff work.

MozReview-Commit-ID: 9F0mcQl7AAX

components/style/dom.rs

@@ -34,7 +34,7 @@ use std::hash::Hash;
 use std::ops::Deref;
 use stylist::Stylist;
 use thread_state;
-use traversal_flags::TraversalFlags;
+use traversal_flags::{TraversalFlags, self};
 
 pub use style_traits::UnsafeNode;
 
@@ -489,6 +489,11 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
                    !data.restyle.hint.has_animation_hint_or_recascade();
         }
 
+        if traversal_flags.contains(traversal_flags::UnstyledOnly) {
+            // We don't process invalidations in UnstyledOnly mode.
+            return data.has_styles();
+        }
+
         if self.has_snapshot() && !self.handled_snapshot() {
             return false;
         }