[web_dev_style] Changing :pseudo-element to ::pseudo-element to match

chrome/browser/resources/chromeos/login/oobe.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 Out of the box experience (OOBE) flow,
  * sign in and lock screens.
  */
 
 html,
 body {
@@ skipping 192 matching lines @@
 .menu-control {
   cursor: default;
 }
 
 #header-sections {
   display: inline-block;
   height: 30px;
   position: relative;
 }
 
-.header-section:before {
+.header-section::before {
   /* Divider in header between product name and title,
      like "[Product name] > [step header]"  */
   content: '\00A0\203A\00A0\00A0';
 }
 
 .header-section {
   -webkit-transition: left 200ms, opacity 200ms ease-in-out;
   position: absolute;
   text-transform: lowercase;
   width: 20em;
@@ skipping 1182 matching lines @@
   width: 100%;
 }
 
 .background-initial {
   opacity: 0;
 }
 
 .background-final {
   opacity: 1;
 }