[ServiceWorker] Treat the rejected result of onFetch event as a Network Error.

In current code the rejected result of onFetch event is treated as 500 respose.
But it should be treated as a Network Error.

Step1: [ServiceWorker] Remove the test for the rejected result of FetchEvent from fetch-event.html
  https://codereview.chromium.org/588993002/
Step2: this patch
  [ServiceWorker] Treat the rejected result of onFetch as a Network Error.
  https://codereview.chromium.org/590043002/
Step3: [ServiceWorker] Add test for the rejected result of FetchEvent using XHR.
  https://codereview.chromium.org/588093003/

BUG=411173

Review URL: https://codereview.chromium.org/590043002

Cr-Commit-Position: refs/heads/master@{#296080}

content/browser/service_worker/service_worker_url_request_job.cc

@@ -18,6 +18,7 @@
 #include "content/common/service_worker/service_worker_types.h"
 #include "content/public/browser/blob_handle.h"
 #include "content/public/browser/resource_request_info.h"
+#include "net/base/net_errors.h"
 #include "net/http/http_request_headers.h"
 #include "net/http/http_response_headers.h"
 #include "net/http/http_response_info.h"
@@ -380,9 +381,10 @@ void ServiceWorkerURLRequestJob::DidDispatchFetchEvent(
   // We should have a response now.
   DCHECK_EQ(SERVICE_WORKER_FETCH_EVENT_RESULT_RESPONSE, fetch_result);
 
-  // Treat a response whose status is 0 as an error.
+  // Treat a response whose status is 0 as a Network Error.
   if (response.status_code == 0) {
-    DeliverErrorResponse();
+    NotifyDone(
+        net::URLRequestStatus(net::URLRequestStatus::FAILED, net::ERR_FAILED));
     return;
   }