Implement Request.context for Fetch API

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

 <script src="../../resources/testharness.js"></script>
 <script src="test-helpers.js?pipe=sub"></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') {
       continue;
@@ skipping 39 matching lines @@
         xhr.withCredentials = true;
       }
       xhr.open(method, url_base + '/dummy?test', true);
       xhr.send(data);
     });
 }
 
 function string_test() {
   return xhr_send(host_info['HTTP_ORIGIN'], 'POST', 'test string', false)
     .then(function(response) {
+        assert_equals(response.context, 'xmlhttprequest');
         assert_equals(response.method, 'POST');
         assert_equals(response.body, 'test string');
       });
 }
 
 function blob_test() {
   return xhr_send(host_info['HTTP_ORIGIN'], 'POST', new Blob(['test blob']),
                   false)
     .then(function(response) {
+        assert_equals(response.context, 'xmlhttprequest');
         assert_equals(response.method, 'POST');
         assert_equals(response.body, 'test blob');
       });
 }
 
 function custom_method_test() {
   return xhr_send(host_info['HTTP_ORIGIN'], 'XXX', 'test string xxx', false)
     .then(function(response) {
         assert_equals(response.method, 'XXX');
         assert_equals(response.body, 'test string xxx');
@@ skipping 96 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>