Add LayoutTest for ServiceWorker's sandbox iframe handling.

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);
@@ skipping 37 matching lines @@
 // iframe later if needed.
 function with_iframe(url) {
   return new Promise(function(resolve) {
       var frame = document.createElement('iframe');
       frame.src = url;
       frame.onload = function() { resolve(frame); };
       document.body.appendChild(frame);
     });
 }
 
+function with_sandboxed_iframe(url, sandbox) {
+  return new Promise(function(resolve) {
+      var frame = document.createElement('iframe');
+      frame.sandbox = sandbox;
+      frame.src = url;
+      frame.onload = function() { resolve(frame); };
+      document.body.appendChild(frame);
+    });
+}
+
 function normalizeURL(url) {
   return new URL(url, self.location).toString().replace(/#.*$/, '');
 }
 
 function wait_for_update(test, registration) {
   if (!registration || registration.unregister == undefined) {
     return Promise.reject(new Error(
       'wait_for_update must be passed a ServiceWorkerRegistration'));
   }
 
@@ skipping 120 matching lines @@
 }
 
 function login_https(test) {
   return test_login(test, 'https://127.0.0.1:8443',
                     'username1s', 'password1s', 'cookie1')
     .then(function() {
         return test_login(test, 'https://localhost:8443',
                           'username2s', 'password2s', 'cookie2');
       });
 }