Remove -webkit-*-before/after in:

ui/login/oobe.css

 /* Copyright 2014 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,
  * sign in and lock screens.
  */
 
 html,
 body {
@@ skipping 208 matching lines @@
   top: 0;
   width: 100%;
 }
 
 .background-initial {
   opacity: 0;
 }
 
 .throbber {
   -webkit-animation: throbber-animation 1s steps(36) infinite;
-  -webkit-margin-before: 2px;
   -webkit-margin-end: 4px;
   background-image: url(chrome://theme/IDR_CROS_DEFAULT_THROBBER);
   display: inline-block;
   height: 16px;
+  margin-top: 2px;
   width: 16px;
 }
 
 @-webkit-keyframes throbber-animation {
   from {
     background-position: 0;
   }
   to {
     background-position: -576px;
   }
 }