style: Refactor the cascade function.

The `cascade_primary_or_pseudo` function was nice when we shared more code, but
right now I think it just makes it harder to understand what's going on.

components/script/layout_wrapper.rs

@@ -404,9 +404,9 @@ impl<'le> TElement for ServoLayoutElement<'le> {
 
     #[inline]
     fn existing_style_for_restyle_damage<'a>(&'a self,
-                                             current_cv: &'a Arc<ComputedValues>,
+                                             current_cv: &'a ComputedValues,
                                              _pseudo_element: Option<&PseudoElement>)
-                                             -> Option<&'a Arc<ComputedValues>> {
+                                             -> Option<&'a ComputedValues> {
         Some(current_cv)
     }