Bug 1610704 - Add missing call to base class' Cancel. r=baku

Differential Revision: https://phabricator.services.mozilla.com/D60660

--HG--
extra : moz-landing-system : lando

dom/notification/Notification.cpp

@@ -366,7 +366,7 @@ class ReleaseNotificationRunnable final : public NotificationWorkerRunnable {
 
   nsresult Cancel() override {
     mNotification->ReleaseObject();
-    return NS_OK;
+    return NotificationWorkerRunnable::Cancel();
   }
 };