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

LayoutTests/http/tests/serviceworker/resources/fetch-rewrite-worker.js

 function get_query_params(url) {
   var search = (new URL(url)).search;
   if (!search) {
     return {};
   }
   var ret = {};
   var params = search.substring(1).split('&');
   params.forEach(function(param) {
       var element = param.split('=');
       ret[decodeURIComponent(element[0])] = decodeURIComponent(element[1]);
@@ skipping 15 matching lines @@
   return init;
 }
 
 self.addEventListener('fetch', function(event) {
     var params = get_query_params(event.request.url);
     var init = get_request_init(params);
     var url = params['url'];
     if (params['ignore']) {
       return;
     }
+    if (params['reject']) {
+      event.respondWith(new Promise(function(resolve, reject) {
+          reject();
+        }));
+      return;
+    }
     event.respondWith(new Promise(function(resolve, reject) {
         var request = event.request;
         if (url) {
           request = new Request(url, init);
         }
         fetch(request).then(resolve, reject);
       }));
   });