Add a helper method to create SW SimpleEventCallbacks.

content/browser/service_worker/service_worker_dispatcher_host.cc

Index: content/browser/service_worker/service_worker_dispatcher_host.cc
diff --git a/content/browser/service_worker/service_worker_dispatcher_host.cc b/content/browser/service_worker/service_worker_dispatcher_host.cc
index f23aece6697dd199ee86bd81e291c93396551376..0c4a3d3e1b40db452625e6b23eff89e59dd6a7eb 100644
--- a/content/browser/service_worker/service_worker_dispatcher_host.cc
+++ b/content/browser/service_worker/service_worker_dispatcher_host.cc
@@ -1240,12 +1240,8 @@ void ServiceWorkerDispatcherHost::
       event->source.service_worker_info.url = GURL();
   }
 
-  // |event_dispatcher| is owned by |worker|, once |worker| got destroyed, the
-  // bound function will never be called, so it is safe to use
-  // base::Unretained() here.
   worker->event_dispatcher()->DispatchExtendableMessageEvent(
-      std::move(event), base::Bind(&ServiceWorkerVersion::OnSimpleEventFinished,
-                                   base::Unretained(worker.get()), request_id));
+      std::move(event), worker->CreateSimpleEventCallback(request_id));
 }
 
 template <typename SourceInfo>