[ServiceWorker] Set FetchRequestMode and handle wasFetchedViaServiceWorker.

LayoutTests/http/tests/serviceworker/resources/test-helpers.js

 // Adapter for testharness.js-style tests with Service Workers
 
 function service_worker_unregister_and_register(test, url, scope) {
   if (!scope || scope.length == 0)
     return Promise.reject(new Error('tests must define a scope'));
 
   var options = { scope: scope };
   return service_worker_unregister(test, scope)
       .then(function() {
           return navigator.serviceWorker.register(url, options);
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         HTTPS_ORIGIN: 'https://' + ORIGINAL_HOST + ':' + HTTPS_PORT,
         HTTP_REMOTE_ORIGIN: 'http://' + REMOTE_HOST + ':' + HTTP_PORT,
         HTTPS_REMOTE_ORIGIN: 'https://' + REMOTE_HOST + ':' + HTTPS_PORT,
         UNAUTHENTICATED_ORIGIN: 'http://' + UNAUTHENTICATED_HOST + ':' + HTTP_PORT
     };
 }
 
 function base_path() {
     return location.pathname.replace(/\/[^\/]*$/, '/');
 }
+
+function test_login(test, origin, username, password) {
+  return new Promise(function(resolve, reject) {
+      with_iframe(
+        origin + base_path() +
+        'resources/fetch-access-control-login.html')
+        .then(test.step_func(function(frame) {
+            var channel = new MessageChannel();
+            channel.port1.onmessage = test.step_func(function() {
+                unload_iframe(frame);
+                resolve();
+              });
+            frame.contentWindow.postMessage(
+              {username: username, password: password},
+              [channel.port2], origin);
+          }));
+    });
+}