Add tests for navigator.serviceWorker.waiting.

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

 // Adapter for testharness.js-style tests with Service Workers
 
 function service_worker_test(url, description) {
     var t = async_test(description);
     t.step(function() {
         var scope = 'nonexistent';
         service_worker_unregister_and_register(t, url, scope, onRegistered);
 
         function onRegistered(worker) {
             var messageChannel = new MessageChannel();
@@ skipping 16 matching lines @@
             return navigator.serviceWorker.register(url, options);
         },
         unreached_rejection(test, 'Unregister should not fail')
     ).then(
         test.step_func(onregister),
         unreached_rejection(test, 'Registration should not fail')
     );
 }
 
 function service_worker_unregister_and_done(test, scope) {
-    navigator.serviceWorker.unregister(scope).then(
+    return navigator.serviceWorker.unregister(scope).then(
         test.done.bind(test),
         unreached_rejection(test, 'Unregister should not fail'));
 }
 
 // Rejection-specific helper that provides more details
 function unreached_rejection(test, prefix) {
     return test.step_func(function(reason) {
         assert_unreached(prefix + ': ' + reason.name);
     });
 }
 
 // FIXME: Clean up the iframe when the test completes.
 function with_iframe(url, f) {
-  var frame = document.createElement('iframe');
-  frame.src = url;
-  frame.onload = function() {
-      f(frame);
-  };
-  document.body.appendChild(frame);
+    return new Promise(function(resolve, reject) {
+        var frame = document.createElement('iframe');
+        frame.src = url;
+        frame.onload = function() {
+            if (f) {
+              f(frame);
+            }
+            resolve(frame);
+        };
+        document.body.appendChild(frame);
+    });
 }
 
 function normalizeURL(url) {
   return new URL(url, document.location).toString().replace(/#.*$/, '');
 }