Don't use ServiceWorkerContainer.unregister in service_worker_unregister_and_register

LayoutTests/http/tests/serviceworker/install-phase-event-waituntil.html

Index: LayoutTests/http/tests/serviceworker/install-phase-event-waituntil.html
diff --git a/LayoutTests/http/tests/serviceworker/install-phase-event-waituntil.html b/LayoutTests/http/tests/serviceworker/install-phase-event-waituntil.html
index fc9258f4558d78799bb7431770b21c438c7acb70..7a504a3ec8a73f529ce789ff83bc193bf9431518 100644
--- a/LayoutTests/http/tests/serviceworker/install-phase-event-waituntil.html
+++ b/LayoutTests/http/tests/serviceworker/install-phase-event-waituntil.html
@@ -4,10 +4,24 @@
 <script src="../resources/testharnessreport.js"></script>
 <script src="resources/test-helpers.js"></script>
 <script>
+// FIXME: Use the test-helpers.js version once it can handle workers that
+// don't install successfully.
+function unregisterAndRegister(test, script, scope) {
+  var empty_worker = 'resources/empty-worker.js';
+  return navigator.serviceWorker.register(empty_worker, {scope: scope})
+    .then(function(registration) {
+        return registration.unregister();
+      })
+    .then(function() {
+        return navigator.serviceWorker.register(script, {scope: scope});
+      })
+    .catch(unreached_rejection(test));
+}
+
 function runTest(test, scope, onRegister) {
   var script = 'resources/install-phase-event-waituntil.js';
   test.step(function() {
-      service_worker_unregister_and_register(test, script, scope)
+      unregisterAndRegister(test, script, scope)
         .then(function(registration) {
             return wait_for_update(test, registration);
           })