Attempt to fix flaky PlatformAppBrowserTest.Messaging.

chrome/test/data/extensions/platform_apps/messaging/app2/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.
 
 var otherId = 'ljhhihhmjomkjokmknellgbidphmahkh';
 
-chrome.test.sendMessage('Launched');
-
 chrome.runtime.onConnectExternal.addListener(function(port) {
   port.onMessage.addListener(function(msg) {
     if (msg == 'ok_to_disconnect') {
       port.disconnect();
     } else {
       port.postMessage(msg + '_reply');
     }
   });
 });
 
 chrome.runtime.onMessageExternal.addListener(function(msg, sender, callback) {
   chrome.test.assertEq({id: otherId}, sender);
   if (msg == 'hello')
     callback('hello_response');
   else
     callback();
 });
+
+// Must ensure that the listeners are active before sending the "Ready"
+// message (which will cause app1 to be launched).
+chrome.test.sendMessage('Ready');