Fix flaky SW registration in PushMessaging/BudgetManager browser tests

chrome/test/data/push_messaging/push_test.js

Index: chrome/test/data/push_messaging/push_test.js
diff --git a/chrome/test/data/push_messaging/push_test.js b/chrome/test/data/push_messaging/push_test.js
index 73907ba5f1124e4b56e410bb4b720a00c39f1463..8bd7ebea1e601b48a4379aa6940926212f6e2920 100644
--- a/chrome/test/data/push_messaging/push_test.js
+++ b/chrome/test/data/push_messaging/push_test.js
@@ -48,10 +48,11 @@ function registerServiceWorker() {
   // The base dir used to resolve service_worker.js and the scope depends on
   // whether this script is included from an html file in ./, subscope1/, or
   // subscope2/.
-  navigator.serviceWorker.register('service_worker.js', {scope: './'}).then(
-      function(swRegistration) {
-        sendResultToTest('ok - service worker registered');
-      }, sendErrorToTest);
+  navigator.serviceWorker.register('service_worker.js', {
+    scope: './'
+  }).then(swRegistrationReady).then(() => {
+    sendResultToTest('ok - service worker registered');
+  }).catch(sendErrorToTest);
 }
 
 function unregisterServiceWorker() {