Implement background sync behaviour for FakeServiceWorker

Change-Id: I0f72337a6007260044e443ef2b4e4a90a0947b6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2395737Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Clovis PJ <clovispj@google.com>
Cr-Commit-Position: refs/heads/master@{#806631}

content/browser/service_worker/fake_service_worker.cc

@@ -172,14 +172,14 @@ void FakeServiceWorker::DispatchSyncEvent(const std::string& tag,
                                           bool last_chance,
                                           base::TimeDelta timeout,
                                           DispatchSyncEventCallback callback) {
-  NOTIMPLEMENTED();
+  std::move(callback).Run(blink::mojom::ServiceWorkerEventStatus::COMPLETED);
 }
 
 void FakeServiceWorker::DispatchPeriodicSyncEvent(
     const std::string& tag,
     base::TimeDelta timeout,
     DispatchPeriodicSyncEventCallback callback) {
-  NOTIMPLEMENTED();
+  std::move(callback).Run(blink::mojom::ServiceWorkerEventStatus::COMPLETED);
 }
 
 void FakeServiceWorker::DispatchAbortPaymentEvent(