script: Mark callback methods with CanGc. (#35753)

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/customelementregistry.rs

@@ -1020,6 +1020,7 @@ impl CustomElementReaction {
                     arguments,
                     value.handle_mut(),
                     ExceptionHandling::Report,
+                    can_gc,
                 );
             },
         }