LibWeb: Don't construct stylesheet when modifying CSSStyleSheet rules

Previously, `CSSStyleSheet.replace()` and `CSSStyleSheet.replaceSync()`
parsed the given CSS text into a temporary stylesheet object, from
which a list of rules was extracted. Doing this had the unintended
side-effect that a fetch request would be started if the given CSS text
referenced any external resources. This fetch request would cause a
crash, since the temporary stylesheet object didn't set the constructed
flag, or constructor document. We now parse the given CSS text as a
list of rules without constructing a temporary stylesheet.

Libraries/LibWeb/CSS/Parser/Parser.h

@@ -100,6 +100,7 @@ public:
     Vector<Descriptor> parse_as_list_of_descriptors(AtRuleID);
     CSSRule* parse_as_css_rule();
     Optional<StyleProperty> parse_as_supports_condition();
+    GC::RootVector<GC::Ref<CSSRule>> parse_as_stylesheet_contents();
 
     enum class SelectorParsingMode {
         Standard,