Flip --enable-apps-show-on-first-paint flag.

chrome/test/data/extensions/platform_apps/web_view/navigation/embedder.js

 // Copyright 2013 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 embedder = {};
 embedder.tests = {};
 
 window.runTest = function(testName) {
   if (!embedder.test.testList[testName]) {
     console.log('Incorrect testName: ' + testName);
@@ skipping 260 matching lines @@
   };
   webview.addEventListener('loadstop', onLoadStop);
   webview.src = embedder.getHTMLForGuestWithTitle_('step1');
 }
 
 embedder.test.testList = {
   'testNavigation': testNavigation,
   'testBackForwardKeys': testBackForwardKeys
 };
 
-onload = function() {
+chrome.app.window.current().onWindowFirstShown.addListener(function() {
   chrome.test.getConfig(function(config) {
     chrome.test.sendMessage("Launched");
   });
-};
+});