MD User manager: Migrate from PaperDialogBehavior to cr-dialog.

chrome/browser/resources/md_user_manager/error_dialog.js

 // 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.
 
 /**
  * @fileoverview 'error-dialog' is a dialog that displays error messages
  * in the user manager.
  */
 (function() {
 Polymer({
   is: 'error-dialog',
 
   properties: {
     /**
      * The message shown in the dialog.
      * @private {string}
      */
     message_: {type: String, value: ''}
   },
 
   /**
    * Displays the dialog populated with the given message.
    * @param {string} message Error message to show.
    */
   show: function(message) {
     this.message_ = message;
-    this.$.dialog.open();
+    this.$.dialog.showModal();
   }
 });
 })();