Three dot menu is shown now on WebView start screen.

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-start: 15px;
   border-bottom: 3px solid black;
   bottom: 0;
   left: 0;
@@ skipping 51 matching lines @@
 
 .header-bar-item {
   display: inline-block;
   height: 34px;
 }
 
 .add-supervised-user-menu {
   display: none;
 }
 
-#more-settings-header-bar-item.active .add-supervised-user-menu {
+#more-settings-header-bar-item.active button.add-supervised-user-menu {
   background-color: white;
   border: 1px solid lightgray;
   border-radius: 2px;
   bottom: 15px;
+  color: black !important;
   display: block;
   font-size: 13px;
   left: 15px;
+  margin: 0 0 -10px -10px;
   position: absolute;
-  width: 220px;
+  text-align: center;
+  width: 180px;
 }
 
 html[dir=rtl] .header-bar-item {
   background-position: right center;
 }
 
 #login-header-bar #shutdown-button,
 #login-header-bar #restart-button,
 #login-header-bar #add-user-button,
 #login-header-bar #guest-user-button,
@@ skipping 41 matching lines @@
 #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;
 }