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/style/dom.rs

@@ -389,7 +389,7 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
     /// values as an argument here, but otherwise Servo would crash due to
     /// double borrows to return it.
     fn existing_style_for_restyle_damage<'a>(&'a self,
-                                             current_computed_values: &'a Arc<ComputedValues>,
+                                             current_computed_values: &'a ComputedValues,
                                              pseudo: Option<&PseudoElement>)
                                              -> Option<&'a PreExistingComputedValues>;