Don't use ServiceWorkerContainer.unregister in service_worker_unregister_and_register

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

Index: LayoutTests/http/tests/serviceworker/resources/test-helpers.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
index 0bae04421a4bb6364659d6bfde02151695ae6d71..bd9f888e9d406f58ded0513e4ad25f0bb12caae0 100644
--- a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
+++ b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
@@ -1,17 +1,19 @@
 // Adapter for testharness.js-style tests with Service Workers
 
+// Can only be used with a worker that installs successfully, since it
+// first registers to acquire a ServiceWorkerRegistration object to
+// unregister.
+// FIXME: Use getRegistration() when implemented.
 function service_worker_unregister_and_register(test, url, scope) {
-    var options = scope ? { scope: scope } : {};
-    return navigator.serviceWorker.unregister(scope).then(
-        test.step_func(function() {
-            return navigator.serviceWorker.register(url, options);
-        }),
-        unreached_rejection(test, 'Unregister should not fail')
-    ).then(test.step_func(function(worker) {
-          return Promise.resolve(worker);
-        }),
-        unreached_rejection(test, 'Registration should not fail')
-    );
+  var options = scope ? { scope: scope } : {};
+  return navigator.serviceWorker.register(url, options)
+      .then(function(registration) {
+          return registration.unregister();
+        })
+      .then(function() {
+          return navigator.serviceWorker.register(url, options);
+        })
+      .catch(unreached_rejection(test));
 }
 
 function service_worker_unregister_and_done(test, scope) {