Introduce a ToCssWithGuard trait

components/script/dom/cssgroupingrule.rs

@@ -14,6 +14,7 @@ use dom::cssstylesheet::CSSStyleSheet;
 use dom_struct::dom_struct;
 use parking_lot::RwLock;
 use std::sync::Arc;
+use style::shared_lock::SharedRwLock;
 use style::stylesheets::CssRules as StyleCssRules;
 
 #[dom_struct]
@@ -40,6 +41,10 @@ impl CSSGroupingRule {
                                                   parent_stylesheet,
                                                   RulesSource::Rules(self.rules.clone())))
     }
+
+    pub fn shared_lock(&self) -> &SharedRwLock {
+        self.cssrule.shared_lock()
+    }
 }
 
 impl CSSGroupingRuleMethods for CSSGroupingRule {