LibWeb: Ensure CSSStyleSheet::css_rules() always returns the same object

Instead of having a method that replaces the stylesheet's CSSRuleList,
we now have a method on CSSRuleList to modify it's underlying CSSRules.

Userland/Libraries/LibWeb/CSS/CSSRuleList.h

@@ -62,6 +62,8 @@ public:
     bool evaluate_media_queries(HTML::Window const&);
     void for_each_effective_keyframes_at_rule(Function<void(CSSKeyframesRule const&)> const& callback) const;
 
+    void set_rules(Badge<CSSStyleSheet>, Vector<JS::NonnullGCPtr<CSSRule>> rules) { m_rules = move(rules); }
+
     Function<void()> on_change;
 
 private:

Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h

@@ -44,7 +44,6 @@ public:
 
     CSSRuleList const& rules() const { return *m_rules; }
     CSSRuleList& rules() { return *m_rules; }
-    void set_rules(CSSRuleList& rules) { m_rules = &rules; }
 
     CSSRuleList* css_rules() { return m_rules; }
     CSSRuleList const* css_rules() const { return m_rules; }