LibWeb: Only update style for the element being animated by CSS

Unlike DOM mutations, CSS animations don't affect the style of the
entire subtree of the element being animated. This means we only have to
recompute style for the animating element, which is significantly
faster than doing the whole subtree.

This takes idle CPU usage on https://shopify.com/ from 100% to 30% on my
(not massively powerful) laptop. :^)

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1325,7 +1325,7 @@ void StyleComputer::ensure_animation_timer() const
                 m_active_animations.remove(key);
 
             for (auto* element : owning_elements_to_invalidate)
-                element->invalidate_style();
+                element->set_needs_style_update(true);
         });
     }