LibWeb: Coalesce layouts that happen in response to style changes

Instead of doing a forced layout synchronously whenever an element's
style is changed, use a zero-timer to do the forced relayout on next
event loop iteration.

This effectively coalesces a lot of layouts and makes many pages such
as GitHub spend way less time doing redundant layout work.

Libraries/LibWeb/DOM/Element.cpp

@@ -221,7 +221,7 @@ void Element::recompute_style()
         return;
     layout_node()->apply_style(*new_specified_css_values);
     if (diff == StyleDifference::NeedsRelayout) {
-        document().force_layout();
+        document().schedule_forced_layout();
         return;
     }
     if (diff == StyleDifference::NeedsRepaint) {