[ServiceWorker] Use the cookie which is differnt from the username in LayoutTest.

LayoutTests/http/tests/serviceworker/resources/fetch-access-control-login.html

 <script>
 // Set authentication info
 window.addEventListener("message", function(evt) {
     var port = evt.ports[0];
-    document.cookie = 'cookie=' + evt.data.username;
+    document.cookie = 'cookie=' + evt.data.cookie;
     var xhr = new XMLHttpRequest();
     xhr.addEventListener('load', function() {
         port.postMessage({msg: 'LOGIN FINISHED'});
       }, false);
     xhr.open('GET',
              './fetch-access-control.php?Auth',
              true,
              evt.data.username, evt.data.password);
     xhr.send();
   }, false);
 </script>