LibWeb: Make CSSImportList mark its document

We don't need to use a WeakPtr here anymore, since the garbage collector
will resolve any cycles.

Userland/Libraries/LibWeb/CSS/CSSImportRule.cpp

@@ -43,6 +43,7 @@ CSSImportRule::CSSImportRule(AK::URL url, DOM::Document& document)
 void CSSImportRule::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
+    visitor.visit(m_document);
     visitor.visit(m_style_sheet);
 }