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

chrome/test/data/extensions/platform_apps/geometry/test.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.app.runtime.onLaunched.addListener(function() {
+  chrome.test.sendMessage('Launched');
+
   var win1;
   chrome.app.window.create('empty.html',
       { id: 'test',
         left: 113, top: 117, width: 314, height: 271,
         frame: 'none' }, function(win) {
     win1 = win;
 
     var bounds = win.getBounds();
 
     // TODO(jeremya): convert to use getBounds() once
@@ skipping 18 matching lines @@
       chrome.test.assertEq(137, bounds.left);
       chrome.test.assertEq(143, bounds.top);
       chrome.test.assertEq(203, bounds.width);
       chrome.test.assertEq(187, bounds.height);
       chrome.test.sendMessage('Done2');
       win.close()
       window.close();
     });
   });
 });