[Service Workers] Follow-up CL for DCHECK from WebServiceWorkerResponse.

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/iso-latin1-header-iframe.html

 <script src="../../resources/testharness.js"></script>
 <script src="test-helpers.js?pipe=sub"></script>
 <script>
 
 function xhr_send(method, data) {
   return new Promise(function(resolve, reject) {
       var xhr = new XMLHttpRequest();
       xhr.onload = function() {
         resolve();
       };
       xhr.onerror = function() {
-        reject('XHR must fail.');
+        reject('XHR must succeed.');
       };
       xhr.responseType = 'text';
       xhr.open(method, './dummy?test', true);
       xhr.send(data);
     });
 }
 
-
 window.addEventListener('message', function(evt) {
     var port = evt.ports[0];
     xhr_send('POST', 'test string')
       .then(function() { port.postMessage({results: 'finish'}); })
       .catch(function(e) { port.postMessage({results: 'failure:' + e}); });
   });
+
 </script>