[ServiceWorker] Add test for the rejected result of FetchEvent using XHR.

LayoutTests/http/tests/serviceworker/resources/fetch-cors-xhr-iframe.html

Index: LayoutTests/http/tests/serviceworker/resources/fetch-cors-xhr-iframe.html
diff --git a/LayoutTests/http/tests/serviceworker/resources/fetch-cors-xhr-iframe.html b/LayoutTests/http/tests/serviceworker/resources/fetch-cors-xhr-iframe.html
index 020742891523629ecbfcd9938caff9d01a147c57..f9c2e184f4d8146cd65c4861504b9e92706857a9 100644
--- a/LayoutTests/http/tests/serviceworker/resources/fetch-cors-xhr-iframe.html
+++ b/LayoutTests/http/tests/serviceworker/resources/fetch-cors-xhr-iframe.html
@@ -42,6 +42,12 @@ window.addEventListener('message', function(evt) {
             false);
         })
       .then(function() {
+          return create_failure_test_promise('./dummy?reject', false);
+        })
+      .then(function() {
+          return create_failure_test_promise('./dummy?resolve-null', false);
+        })
+      .then(function() {
           return create_success_test_promise(
             './dummy?url=' +
              encodeURIComponent(host_info['HTTP_ORIGIN'] + path),