Add a (disabled) test to demonstrate child frame creation race.

chrome/test/data/extensions/platform_apps/app_window_send_message/test.js

Index: chrome/test/data/extensions/platform_apps/app_window_send_message/test.js
diff --git a/chrome/test/data/extensions/platform_apps/app_window_send_message/test.js b/chrome/test/data/extensions/platform_apps/app_window_send_message/test.js
new file mode 100644
index 0000000000000000000000000000000000000000..5dad5a1d1e12fbc4f63ce1e11cda9a971eb43bc4
--- /dev/null
+++ b/chrome/test/data/extensions/platform_apps/app_window_send_message/test.js
@@ -0,0 +1,12 @@
+// Copyright 2016 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.app.window.create('main.html', {}, function (win) {
+    // Send the message PlatformAppBrowserTest.AppWindowIframe is waiting for.
+    // This is the last step the test does and this step will cause the test
+    // to shut down.
+    chrome.test.sendMessage('APP_WINDOW_CREATE_CALLBACK');
+  });
+});