Fix flaky SW registration in PushMessaging/BudgetManager browser tests

chrome/test/data/push_messaging/service_worker.js

Index: chrome/test/data/push_messaging/service_worker.js
diff --git a/chrome/test/data/push_messaging/service_worker.js b/chrome/test/data/push_messaging/service_worker.js
index f32068849a8fb2b3995e84fd895d82133efc97a0..53cc8bcf8312330dbb34fe9dcbe2104086f1dcc7 100644
--- a/chrome/test/data/push_messaging/service_worker.js
+++ b/chrome/test/data/push_messaging/service_worker.js
@@ -6,6 +6,9 @@
 // which would affect the scope of the importScripts call here.
 self.importScripts('/push_messaging/push_constants.js');
 
+// Don't wait for clients of old SW to close before activating.
+self.addEventListener('install', () => skipWaiting());
+
 var pushSubscriptionOptions = {
     userVisibleOnly: true
 };