Service Worker: Add tests for same-scope new script registration

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 6bed2f1460ccb43a2f7b11e2e5749747b113cfbe..f72f60c71e4f53ebbe2332fdc96891d31ada3cf6 100644
--- a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
+++ b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
@@ -66,18 +66,28 @@ function normalizeURL(url) {
 }
 
 function wait_for_update(test, registration) {
-    return new Promise(test.step_func(function(resolve) {
-        registration.addEventListener('updatefound', test.step_func(function() {
-            resolve(registration.installing);
-        }));
+  if (!registration || registration.unregister == undefined) {
+    return Promise.reject(new Error(
+      'wait_for_update must be passed a ServiceWorkerRegistration'));
+  }
+
+  return new Promise(test.step_func(function(resolve) {
+      registration.addEventListener('updatefound', test.step_func(function() {
+          resolve(registration.installing);
+      }));
     }));
 }
 
 function wait_for_state(test, worker, state) {
-    return new Promise(test.step_func(function(resolve) {
-        worker.addEventListener('statechange', test.step_func(function() {
-            if (worker.state === state)
-                resolve(state);
+  if (!worker || worker.state == undefined) {
+    return Promise.reject(new Error(
+      'wait_for_state must be passed a ServiceWorker'));
+  }
+
+  return new Promise(test.step_func(function(resolve) {
+      worker.addEventListener('statechange', test.step_func(function() {
+          if (worker.state === state)
+            resolve(state);
         }));
     }));
 }