LibWeb: Evaluate @media rules

We now evaluate the conditions of `@media` rules at the same point in
the HTML event loop as evaluation of `MediaQueryList`s. This is not
strictly to spec, but since the spec doesn't actually say when to do
this, it seems to make the most sense. In any case, it works! :^)

Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h

@@ -46,6 +46,7 @@ public:
 
     void for_each_effective_style_rule(Function<void(CSSStyleRule const&)> const& callback) const;
     bool for_first_not_loaded_import_rule(Function<void(CSSImportRule&)> const& callback);
+    void evaluate_media_queries(DOM::Window const&);
 
 private:
     explicit CSSStyleSheet(NonnullRefPtrVector<CSSRule>);