Fix flaky SW registration in PushMessaging/BudgetManager browser tests

chrome/test/data/budget_service/service_worker.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// Don't wait for clients of old SW to close before activating.
+self.addEventListener('install', () => skipWaiting());
+
 // Accept messages from the test JavaScript to trigger worker based tests.
 self.addEventListener('message', function (event) {
   if (event.data.command == 'workerGet') {
     workerGetBudget();
   } else if (event.data.command == 'workerReserve') {
     workerReserve();
   } else {
     sendMessageToClients('message', 'error - unknown message request');
     return;
   }
@@ skipping 28 matching lines @@
     'data': data
   });
   clients.matchAll().then(function(clients) {
     clients.forEach(function(client) {
       client.postMessage(message);
     });
   }, function(error) {
     console.log(error);
   });
 }