stylo: Remove a lot of the restyle damage related complexity.

The only reason why we had the `existing_style_for_style_damage` bit is to apply
some optimizations that we don't have anymore.

I still want to reintroduce a few of them, at least for the non-eager
pseudo-element case... But I think I won't need this at all.

This allows us to remove a fair amount of Gecko code too.

components/layout/animation.rs

@@ -161,9 +161,10 @@ pub fn recalc_style_for_animations(context: &LayoutContext,
                                            &mut fragment.style,
                                            &ServoMetricsProvider);
                 let difference =
-                    RestyleDamage::compute_style_difference(&old_style,
-                                                            &old_style,
-                                                            &fragment.style);
+                    RestyleDamage::compute_style_difference(
+                        &old_style,
+                        &fragment.style,
+                    );
                 damage |= difference.damage;
             }
         }