Promisify Service Worker test unregister_and_register helper function

LayoutTests/http/tests/serviceworker/state.html

Index: LayoutTests/http/tests/serviceworker/state.html
diff --git a/LayoutTests/http/tests/serviceworker/state.html b/LayoutTests/http/tests/serviceworker/state.html
index a73afa9cb4f802be2e09feef278654e7a313f6a1..db5222e89677db3965c1b6a26bbf922cbec15725 100644
--- a/LayoutTests/http/tests/serviceworker/state.html
+++ b/LayoutTests/http/tests/serviceworker/state.html
@@ -9,8 +9,7 @@
     var currentState = 'test-is-starting';
     var scope = '/state/*';
 
-    service_worker_unregister_and_register(
-        t, 'resources/worker-no-op.js', scope, onRegister);
+    service_worker_unregister_and_register(t, 'resources/worker-no-op.js', scope).then(t.step_func(onRegister));
 
     function onRegister(sw) {
         sw.addEventListener('statechange', t.step_func(onStateChange(sw)));