LibWeb: Make CSSRule and all its subclasses GC-allocated

Author: https://github.com/awesomekling
Commit: 12042f0757
Pull-request: https://github.com/SerenityOS/serenity/pull/14816
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -21,7 +21,7 @@
 namespace Web::CSS {
 
 struct MatchingRule {
-    RefPtr<CSSStyleRule> rule;
+    CSSStyleRule const* rule { nullptr };
     size_t style_sheet_index { 0 };
     size_t rule_index { 0 };
     size_t selector_index { 0 };