<webview>: Fix Javascript error on attributeChanged callback

chrome/renderer/resources/extensions/web_view.js

 // Copyright (c) 2012 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.
 
 // Shim that simulates a <webview> tag via Mutation Observers.
 //
 // The actual tag is implemented via the browser plugin. The internals of this
 // are hidden via Shadow DOM.
 
 'use strict';
@@ skipping 940 matching lines @@
 
 // Registers <webview> custom element.
 function registerWebViewElement() {
   var proto = Object.create(HTMLElement.prototype);
 
   proto.createdCallback = function() {
     new WebViewInternal(this);
   };
 
   proto.attributeChangedCallback = function(name, oldValue, newValue) {
+    if (!this.internal_) {
+      return;
+    }
     var internal = this.internal_(secret);
     internal.handleWebviewAttributeMutation_(name, oldValue, newValue);
   };
 
   proto.back = function() {
     this.go(-1);
   };
 
   proto.forward = function() {
     this.go(1);
@@ skipping 75 matching lines @@
  * Implemented when the experimental API is available.
  * @private
  */
 WebViewInternal.prototype.maybeGetExperimentalPermissions_ = function() {
   return [];
 };
 
 exports.WebView = WebView;
 exports.WebViewInternal = WebViewInternal;
 exports.CreateEvent = CreateEvent;