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

chrome/test/data/extensions/platform_apps/windows_api_bounds/background.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");
   chrome.app.window.create("main.html", function(win){
     // Make sure we get back a bounds and that it isn't all 0's.
     var bounds = win.getBounds();
     if (!bounds ||
         (bounds.top == 0 && bounds.left == 0 &&
          bounds.width == 0 && bounds.height == 0)) {
       console.error("invalid bounds after app.window.create:" +
                     JSON.stringify(bounds));
       win.close();
       window.close();
     } else {
       chrome.test.sendMessage("background_ok");
     }
   });
 });