Fix the WebUIWebViewBrowserTest.AddContentScriptsWithNewWindowAPI.

chrome/test/data/webui/webview_execute_script_test.js

 // Copyright 2015 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 REQUEST_TO_COMM_CHANNEL_1 = 'connect';
 var REQUEST_TO_COMM_CHANNEL_2 = 'connect_request';
 var RESPONSE_FROM_COMM_CHANNEL_1 = 'connected';
 var RESPONSE_FROM_COMM_CHANNEL_2 = 'connected_response';
 
 function createWebview() {
@@ skipping 329 matching lines @@
       newwebview.contentWindow.postMessage(JSON.stringify(msg), '*');
     });
 
     document.body.appendChild(newwebview);
     // attach the new window to the new <webview>.
     console.log('Step 3: attaches the new webview.');
     e.window.attach(newwebview);
   });
 
   window.addEventListener('message', function(e) {
-    if (e.source != newwebview.contentWindow)
+    if (!newwebview || e.source != newwebview.contentWindow)
       return;
     var data = JSON.parse(e.data);
     if (data == RESPONSE_FROM_COMM_CHANNEL_1 &&
         e.source == newwebview.contentWindow) {
       console.log('Step 5: a communication channel has been established ' +
           'with the new webview.');
       chrome.send('testResult', [true]);
       return;
     } else {
       chrome.send('testResult', [false]);
@@ skipping 76 matching lines @@
       console.log('Step 5: check the result of content script injected again.');
       webview.executeScript({
         code: 'document.body.style.backgroundColor;'
       }, onGetBackgroundExecuted);
     }
   });
 
   webview.src = url;
   document.body.appendChild(webview);
 }