Set the request mode and the credentials mode of FetchEvent in the service worker correctly.

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/fetch-request-xhr-iframe.html

 <script src="../../resources/testharness.js"></script>
 <script src="../../resources/get-host-info.js?pipe=sub"></script>
 <script src="test-helpers.js"></script>
 <script>
 var host_info = get_host_info();
 
 function get_boundary(headers) {
   var reg = new RegExp('multipart\/form-data; boundary=(.*)');
   for (var i = 0; i < headers.length; ++i) {
     if (headers[i][0] != 'content-type') {
@@ skipping 115 matching lines @@
           'fs file content\r\n' +
           '--' + boundary + '--\r\n';
         assert_equals(response.body, expected_body);
       });
 }
 
 function mode_credentials_test() {
   return xhr_send(host_info['HTTP_ORIGIN'], 'GET', '', false)
     .then(function(response){
         assert_equals(response.mode, 'cors');
-        assert_equals(response.credentials, 'same-origin');
+        assert_equals(response.credentials, 'include');
         return xhr_send(host_info['HTTP_ORIGIN'], 'GET', '', true);
       })
     .then(function(response){
         assert_equals(response.mode, 'cors');
         assert_equals(response.credentials, 'include');
         return xhr_send(host_info['HTTP_REMOTE_ORIGIN'], 'GET', '', false);
       })
     .then(function(response){
         assert_equals(response.mode, 'cors');
         assert_equals(response.credentials, 'same-origin');
@@ skipping 29 matching lines @@
       .then(blob_test)
       .then(custom_method_test)
       .then(options_method_test)
       .then(form_data_test)
       .then(mode_credentials_test)
       .then(data_url_test)
       .then(function() { port.postMessage({results: 'finish'}); })
       .catch(function(e) { port.postMessage({results: 'failure:' + e}); });
   });
 </script>