Don't force display the addUser button. Also, remove cancelAdduser.

chrome/browser/resources/user_manager/control_bar.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.
 
 /**
  * @fileoverview Desktop User Chooser UI control bar implementation.
  */
 
 cr.define('login', function() {
   /**
@@ skipping 12 matching lines @@
     // Current UI state of the sign-in screen.
     signinUIState_: SIGNIN_UI_STATE.HIDDEN,
 
     // Whether to show kiosk apps menu.
     hasApps_: false,
 
     /** @override */
     decorate: function() {
       $('add-user-button').addEventListener('click',
           this.handleAddUserClick_);
-      $('cancel-add-user-button').addEventListener('click',
-          this.handleCancelAddUserClick_);
       $('guest-user-header-bar-item').addEventListener('click',
           this.handleGuestClick_);
       $('guest-user-button').addEventListener('click',
           this.handleGuestClick_);
       this.updateUI_();
     },
 
     /**
      * Tab index value for all button elements.
      * @type {number}
@@ skipping 83 matching lines @@
     set allowCancel(value) {
       this.allowCancel_ = value;
       this.updateUI_();
     },
 
     /**
      * Updates visibility state of action buttons.
      * @private
      */
     updateUI_: function() {
-      $('add-user-button').hidden = false;

[Nikita (slow), 2014/09/11 20:56:16]

Where would add user button be hidden then?

[Mike Lerman, 2014/09/11 21:05:21]

In showUserManagerScreen of user_manager.js as
perhttps://codereview.chromium.org/518063002/

-      $('cancel-add-user-button').hidden = !this.allowCancel_;
       $('guest-user-header-bar-item').hidden = false;
       $('add-user-header-bar-item').hidden = false;
     },
 
     /**
      * Animates Header bar to hide from the screen.
      * @param {function()} callback will be called once animation is finished.
      */
     animateOut: function(callback) {
       var launcher = this;
@@ skipping 28 matching lines @@
    * Convenience wrapper of animateIn.
    */
   HeaderBar.animateIn = function() {
     $('login-header-bar').animateIn();
   };
 
   return {
     HeaderBar: HeaderBar
   };
 });