ChromeOS OOBE: Fix keyboard focusing of OOBE buttons.

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

 /* Copyright 2016 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. */
 
 .vertical-margin {
-  width: 24px;
+  width: 22px; /* = 24 - (button border 2px) */
 }
 
 #bottom-margin {
   height: 24px;
 }
 
 #title {
   color: rgba(0, 0, 0, 0.87);
   font-family: Roboto, sans-serif;
   font-size: 38px;
   font-weight: normal;
   margin: 120px 0 30px 0;
 }
 
 #welcomeIllustration {
   height: 300px;
   width: 390px;
 }
 
 oobe-welcome-secondary-button {
   margin-right: 8px;
 }
 
 #buttons {
   width: 100%;
 }