[ServiceWorker] Fix a copy/paste miss

For Unregister related codes:
kServiceWorkerUpdateErrorPrefix --> kServiceWorkerUnregisterErrorPrefix

BUG=775412

Change-Id: Ib2a0dff68caef88cacd7ce6d449a7ef6adb4a8c6
Reviewed-on: https://chromium-review.googlesource.com/724439Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#509639}

content/browser/service_worker/service_worker_dispatcher_host.cc

@@ -270,7 +270,7 @@ void ServiceWorkerDispatcherHost::OnUnregisterServiceWorker(
     case ProviderStatus::DEAD_HOST:
       Send(new ServiceWorkerMsg_ServiceWorkerUnregistrationError(
           thread_id, request_id, blink::mojom::ServiceWorkerErrorType::kAbort,
-          base::ASCIIToUTF16(kServiceWorkerUpdateErrorPrefix) +
+          base::ASCIIToUTF16(kServiceWorkerUnregisterErrorPrefix) +
               base::ASCIIToUTF16(kShutdownErrorMessage)));
       return;
     case ProviderStatus::NO_HOST: