Remove -webkit-*-before/after in:

chrome/browser/resources/chromeos/login/network_dropdown.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.
  *
  * This is the stylesheet used by the network drop-down control.
  */
 
 .dropdown-title {
   -webkit-appearance: none;
   -webkit-box-align: start;
@@ skipping 110 matching lines @@
   padding-top: 4px;
   text-overflow: ellipsis;
   white-space: nowrap;
 }
 
 .dropdown-divider {
   height: 3px;
 }
 
 .dropdown-divider hr {
-  -webkit-margin-after: 2px;
-  -webkit-margin-before: 2px;
+  margin-bottom: 2px;
+  margin-top: 2px;
 }
 
 .dropdown-item.disabled-item {
   color: graytext;
 }
 
 .dropdown-overlay {
   bottom: 0;
   left: 0;
   position: fixed;
   right: 0;
   top: 0;
   z-index: 5;
 }