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

chrome/browser/resources/shared/css/tree.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. */
 
 tree {
   display: block;
   outline: none;
   overflow: auto;
 }
 
@@ skipping 131 matching lines @@
       url('../../../../../ui/resources/default_200_percent/common/folder_closed.png') 2x);
 }
 
 .tree-item[expanded] > .tree-row > .tree-label {
   background-image: -webkit-image-set(
       url('../../../../../ui/resources/default_100_percent/common/folder_open.png') 1x,
       url('../../../../../ui/resources/default_200_percent/common/folder_open.png') 2x);
 }
 
 /* We need to ensure that even empty labels take up space */
-.tree-label:empty:after {
+.tree-label:empty::after {
   content: ' ';
   white-space: pre;
 }
 
 .tree-rename > .tree-row > .tree-label {
   -webkit-user-modify: read-write-plaintext-only;
   -webkit-user-select: auto;
   background: white;
   color: black;
   outline: 1px solid black;
 }
 
 .tree-item[editing] input {
   /* Do not inherit the line-height */
   font-family: inherit;
   font-size: inherit;
   font-weight: inherit;
   margin: -2px -8px -2px -3px;
 <if expr="not is_macosx">
   outline: none;
 </if>
   padding: 1px 7px 1px 1px;
 }
 
 html[dir=rtl] .tree-item[editing] input {
   margin: -2px -3px -2px -8px;
   padding: 1px 1px 1px 7px;
 }