LibWeb: Remove CSS::Parser::ParsingContext's default constructor

This relied on pulling the current realm from the main thread VM, which
requires an execution context to be on the VM's stack. This heavily
relied on the dummy execution context that is always on the stack, for
example, when parsing the UA style sheets where no JavaScript is
running.

Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp

@@ -54,7 +54,8 @@ WebIDL::ExceptionOr<unsigned> CSSStyleSheet::insert_rule(StringView rule, unsign
     // FIXME: 2. If the disallow modification flag is set, throw a NotAllowedError DOMException.
 
     // 3. Let parsed rule be the return value of invoking parse a rule with rule.
-    auto parsed_rule = parse_css_rule(CSS::Parser::ParsingContext {}, rule);
+    auto context = m_style_sheet_list ? CSS::Parser::ParsingContext { m_style_sheet_list->document() } : CSS::Parser::ParsingContext { realm() };
+    auto parsed_rule = parse_css_rule(context, rule);
 
     // 4. If parsed rule is a syntax error, return parsed rule.
     if (!parsed_rule)