Add a helper method to create SW SimpleEventCallbacks.

content/browser/service_worker/service_worker_version.cc

Index: content/browser/service_worker/service_worker_version.cc
diff --git a/content/browser/service_worker/service_worker_version.cc b/content/browser/service_worker/service_worker_version.cc
index 8f439bbe390e1200de64bf0eb605f0e137ea27f7..c205b9c9cb349bc49f8672e680a4f585f3d17c77 100644
--- a/content/browser/service_worker/service_worker_version.cc
+++ b/content/browser/service_worker/service_worker_version.cc
@@ -617,6 +617,14 @@ bool ServiceWorkerVersion::FinishExternalRequest(
   return true;
 }
 
+ServiceWorkerVersion::SimpleEventCallback
+ServiceWorkerVersion::CreateSimpleEventCallback(int request_id) {
+  // The weak reference to |this| is safe because storage of the callbacks, the
+  // pending responses of the ServiceWorkerEventDispatcher, is owned by |this|.
+  return base::Bind(&ServiceWorkerVersion::OnSimpleEventFinished,
+                    base::Unretained(this), request_id);
+}
+
 void ServiceWorkerVersion::RunAfterStartWorker(
     ServiceWorkerMetrics::EventType purpose,
     const base::Closure& task,