Keep browser process alive while there are platform apps with background pages running.

chrome/test/data/extensions/platform_apps/open_link/main.js

 // Copyright (c) 2012 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.
 
 chrome.test.getConfig(function(config) {
   var linkNode = document.getElementById('test-link');
   linkNode.href = 'http://localhost:' + config.testServer.port +
       '/files/extensions/platform_apps/open_link/link.html';
 
   var clickEvent = document.createEvent('MouseEvents');
   clickEvent.initMouseEvent('click', true, true, window,
                             0, 0, 0, 0, 0, false, false,
                             false, false, 0, null);
   linkNode.dispatchEvent(clickEvent);
 });
 
 onmessage = function() {
   chrome.test.sendMessage('Link opened');
 };
+
+chrome.test.sendMessage('Launched');