Change lazily_compute_pseudo_element_style to take an &Arc for the parent style.

This makes it easier to do later changes that want to pass an Arc here.

Part 1 of the fix for Gecko bug 1364242: https://bugzilla.mozilla.org/show_bug.cgi?id=1364242

components/style/stylist.rs

@@ -679,7 +679,7 @@ impl Stylist {
                                                   element: &E,
                                                   pseudo: &PseudoElement,
                                                   rule_inclusion: RuleInclusion,
-                                                  parent_style: &ComputedValues,
+                                                  parent_style: &Arc<ComputedValues>,
                                                   font_metrics: &FontMetricsProvider)
                                                   -> Option<Arc<ComputedValues>>
         where E: TElement,