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/CSSGroupingRule.h

@@ -22,6 +22,7 @@ public:
     ~CSSGroupingRule();
 
     CSSRuleList const& css_rules() const { return m_rules; }
+    CSSRuleList& css_rules() { return m_rules; }
     size_t insert_rule(StringView const& rule, size_t index = 0);
     void delete_rule(size_t index);