[chromedriver] Fallback to devtools for screenshotting of forbidden urls.

chrome/test/chromedriver/extension/background.js

 // Copyright (c) 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.
 
 /*
  * Checks for an extension error that occurred during the asynchronous call.
  * If an error occurs, will invoke the error callback and throw an exception.
  *
  * @param {function(!Error)} errCallback The callback to invoke for error
  *     reporting.
  */
 function checkForExtensionError(errCallback) {
   if (typeof(chrome.extension.lastError) != 'undefined') {
     var error = new Error(chrome.extension.lastError.message);
     errCallback(error);
     throw error;
   }
 }
 
 /**
  * Captures a screenshot of the visible tab.
  *
  * @param {function(string)} callback The callback to invoke with the base64
  *     encoded PNG.
  * @param {function(!Error)} errCallback The callback to invoke for error
  *     reporting.
  */
 function captureScreenshot(callback, errCallback) {
   chrome.tabs.captureVisibleTab({format:'png'}, function(dataUrl) {
+    if (chrome.extension.lastError &&
+        chrome.extension.lastError.message.indexOf('permission') != -1) {
+      var error = new Error(chrome.extension.lastError.message);
+      error.code = 103;  // kForbidden
+      errCallback(error);
+      return;
+    }
     checkForExtensionError(errCallback);
     var base64 = ';base64,';
     callback(dataUrl.substr(dataUrl.indexOf(base64) + base64.length))
   });
 }
 
 /**
  * Gets info about the current window.
  *
  * @param {function(*)} callback The callback to invoke with the window info.
@@ skipping 17 matching lines @@
  */
 function updateWindow(updateInfo, callback, errCallback) {
   chrome.windows.getCurrent({}, function(window) {
     checkForExtensionError(errCallback);
     chrome.windows.update(window.id, updateInfo, function(window) {
       checkForExtensionError(errCallback);
       callback();
     });
   });
 }