LibWeb: Don't compute style *again* for elements in Layout::TreeBuilder

TreeBuilder wasn't taking advantage of the fact that we already have
computed style cached on each DOM::Element by the time we're
constructing a layout tree.

So instead of using the cached style, we recomputed it from scratch for
every element. This was done because invalidation was broken in many
places, but now that it's more or less trustworthy, stop recomputing
style on the fly in TreeBuilder and use what the preceding style update
pass gave us instead.

This basically cuts style computation work in half. :^)

Userland/Libraries/LibWeb/Layout/TreeBuilder.cpp

@@ -101,10 +101,10 @@ void TreeBuilder::create_layout_tree(DOM::Node& dom_node, TreeBuilder::Context&
     if (is<DOM::Element>(dom_node)) {
         auto& element = static_cast<DOM::Element&>(dom_node);
         element.clear_pseudo_element_nodes({});
-        style = style_computer.compute_style(element);
+        VERIFY(!element.needs_style_update());
+        style = element.computed_css_values();
         if (style->display().is_none())
             return;
-        element.set_computed_css_values(style);
         layout_node = element.create_layout_node(*style);
     } else if (is<DOM::Document>(dom_node)) {
         style = style_computer.create_document_style();