Attempt to fix flaky PlatformAppBrowserTest.Messaging.

chrome/test/data/extensions/platform_apps/messaging/app2/background.js

Index: chrome/test/data/extensions/platform_apps/messaging/app2/background.js
diff --git a/chrome/test/data/extensions/platform_apps/messaging/app2/background.js b/chrome/test/data/extensions/platform_apps/messaging/app2/background.js
index 6f05fedbd23753c0cae0bf2e3bcfbb18b67727ae..e078fa356e49f5dd705d3038b990044b089aeaf0 100644
--- a/chrome/test/data/extensions/platform_apps/messaging/app2/background.js
+++ b/chrome/test/data/extensions/platform_apps/messaging/app2/background.js
@@ -4,8 +4,6 @@
 
 var otherId = 'ljhhihhmjomkjokmknellgbidphmahkh';
 
-chrome.test.sendMessage('Launched');
-
 chrome.runtime.onConnectExternal.addListener(function(port) {
   port.onMessage.addListener(function(msg) {
     if (msg == 'ok_to_disconnect') {
@@ -23,3 +21,7 @@ chrome.runtime.onMessageExternal.addListener(function(msg, sender, callback) {
   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');