Added login screen mode for adding users into session.

chrome/browser/resources/chromeos/login/header_bar.css

 /* 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.
  */
 
 #login-header-bar {
   -webkit-padding-after: 7px;
   bottom: 0;
   left: 0;
   position: absolute;
@@ skipping 51 matching lines @@
   display: inline-block;
   height: 34px;
 }
 
 html[dir=rtl] .header-bar-item {
   background-position: right center;
 }
 
 #login-header-bar #shutdown-button,
 #login-header-bar #add-user-button,
-#login-header-bar #guest-user-button {
+#login-header-bar #guest-user-button,
+#login-header-bar #cancel-multiple-sign-in-button {
   -webkit-padding-start: 24px;
   background-position: left center;
   background-repeat: no-repeat;
   background-size: 24px;
 }
 
 html[dir=rtl] #login-header-bar #shutdown-button,
 html[dir=rtl] #login-header-bar #add-user-button,
-html[dir=rtl] #login-header-bar #guest-user-button {
+html[dir=rtl] #login-header-bar #guest-user-button,
+html[dir=rtl] #login-header-bar #cancel-multiple-sign-in-button {
   background-position: right center;
 }
 
 #login-header-bar #shutdown-button {
   background-image: url('chrome://theme/IDR_ICON_POWER_WHITE');
 }
 
 #login-header-bar #add-user-button {
   background-image: url('chrome://theme/IDR_ICON_ADD_USER_WHITE');
 }
 
 #login-header-bar #guest-user-button {
   background-image: url('chrome://theme/IDR_ICON_GUEST_WHITE');
 }
 
+#login-header-bar #cancel-multiple-sign-in-button {
+  /* TODO(dzhioev): replace with appropriate image when possible. */
+  background-image: url('chrome://theme/IDR_PANEL_CLOSE');
+}
+
 .button-restricted {
   border: 1px solid transparent;
   color: white !important;
   opacity: 0.4 !important;
 }