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

chrome/browser/resources/performance_monitor/chart.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. */
 
 /* "Performance" title */
 h1 {
   -webkit-margin-after: 1em;
   -webkit-margin-before: 21px;
   -webkit-margin-start: 23px;
   -webkit-user-select: none;
@@ skipping 52 matching lines @@
   -webkit-margin-start: 23px;
   -webkit-user-select: none;
   color: #777;
   cursor: default;
 }
 
 summary:focus {
   outline: 0;
 }
 
-summary:after {
+summary::after {
   background: white;
   content: '';
   height: 32px;
   left: 0;
   margin-top: -23px;
   position: absolute;
   width: 6px;
 }
 
 summary:hover {
   color: rgb(70, 78, 90);
 }
 
-details[open] summary:after {
+details[open] summary::after {
   background: #555;
 }
 
 details[open] summary {
   color: #333;
 }
 
 details summary::-webkit-details-marker {
   display: none;
 }
@@ skipping 101 matching lines @@
   margin: 0 1px 0 0;
   min-width: 4em;
   text-shadow: #F0F0F0 0 1px 0;
 }
 
 button:active {
   background-image: -webkit-linear-gradient(#e7e7e7, #e7e7e7 38%, #d7d7d7);
   box-shadow: none;
   text-shadow: none;
 }