LibWeb: Skip full document style update in getComputedStyle if possible

Before calling update_style() for a getComputedStyle property access,
we now check whether the target element actually needs a style update
by walking the flat tree ancestor chain. If neither the element nor any
of its ancestors have dirty style bits, and there are no document-level
reasons to recalculate style, we skip the update_style() call entirely.

We walk the flat tree (not the DOM tree) because style inheritance
follows slot assignment -- slotted elements inherit from their assigned
slot, not their DOM parent.

This avoids unnecessary work when scripts access computed style
properties on elements whose styles are already up-to-date, which is a
common pattern on the web.

Libraries/LibWeb/CSS/CSSStyleProperties.cpp

@@ -560,7 +560,7 @@ Optional<StyleProperty> CSSStyleProperties::get_direct_property(PropertyNameAndI
             // always need update_layout() to ensure both style and layout tree are up to date.
             abstract_element.document().update_layout(DOM::UpdateLayoutReason::ResolvedCSSStyleDeclarationProperty);
             layout_node = abstract_element.layout_node();
-        } else {
+        } else if (abstract_element.document().element_needs_style_update(abstract_element)) {
             // Just ensure styles are up to date.
             abstract_element.document().update_style();
         }