[cros,webui] Adds scrollbar for the network drop-down controller.

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

 /* Copyright (c) 2011 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.
  *
  * This is the stylesheet used by the Out of the box experience (OOBE) flow.
  */
 
 html,
 body {
   height: 100%;
@@ skipping 636 matching lines @@
   background-repeat: no-repeat;
 }
 
 html[dir=rtl] .header-bar-item,
 html[dir=rtl] #header,
 html[dir=rtl] #login-header-bar #shutdown-button,
 html[dir=rtl] #login-header-bar #add-user-button {
   background-position: right center;
 }
 
-body.login-display #oobe hr {
+body.login-display #oobe hr.shadow {
   -webkit-transition: all .8s ease;
   width: 1100px;  /* Necessary. Otherwise hr shrinks to 0 first when switching
                      from 640px to full width. */
 }
 
 body.login-display #oobe.signin hr,
 body.login-display #oobe.gaia-signin hr {
   opacity: 0;
 }
 
@@ skipping 48 matching lines @@
   left: 10px;
   opacity: 1.0;
   position: absolute;
   right: 10px;
   text-shadow: 0 1px 1px #fff;
 }
 
 #offline-network-control {
   margin-left: 60px;
 }