LibWeb: Skip keyframe animation update if target element isn't connected

Makes some pages on Github faster as previously we had to update
animation for elements in not connected shadow trees.

Libraries/LibWeb/Animations/KeyframeEffect.cpp

@@ -918,7 +918,7 @@ static CSS::RequiredInvalidationAfterStyleChange compute_required_invalidation(H
 void KeyframeEffect::update_computed_properties()
 {
     auto target = this->target();
-    if (!target)
+    if (!target || !target->is_connected())
         return;
 
     GC::Ptr<CSS::ComputedProperties> style = {};