Promisify Service Worker test unregister_and_register helper function

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);
+        service_worker_unregister_and_register(t, url, scope).then(t.step_func(onRegistered));
 
         function onRegistered(worker) {
             var messageChannel = new MessageChannel();
             messageChannel.port1.onmessage = t.step_func(onMessage);
             worker.postMessage({port:messageChannel.port2}, [messageChannel.port2]);
         }
 
         function onMessage(e) {
             assert_equals(e.data, 'pass');
             service_worker_unregister_and_done(t, scope);
         }
     });
 }
 
-function service_worker_unregister_and_register(test, url, scope, onregister) {
+function service_worker_unregister_and_register(test, url, scope) {
     var options = scope ? { scope: scope } : {};
     return navigator.serviceWorker.unregister(scope).then(
-        // FIXME: Wrap this with test.step_func once testharness.js is updated.
-        function() {
+        test.step_func(function() {
             return navigator.serviceWorker.register(url, options);
-        },
+        }),
         unreached_rejection(test, 'Unregister should not fail')
-    ).then(
-        test.step_func(onregister),
+    ).then(test.step_func(function(worker) {
+          return Promise.resolve(worker);
+        }),
         unreached_rejection(test, 'Registration should not fail')
     );
 }
 
 function service_worker_unregister_and_done(test, scope) {
     return navigator.serviceWorker.unregister(scope).then(
         test.done.bind(test),
         unreached_rejection(test, 'Unregister should not fail'));
 }
 
@@ skipping 15 matching lines @@
             }
             resolve(frame);
         };
         document.body.appendChild(frame);
     });
 }
 
 function normalizeURL(url) {
   return new URL(url, document.location).toString().replace(/#.*$/, '');
 }