WebUI: Fix radio and checkbox behavior when text selected.

ui/webui/resources/css/widgets.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 file defines styles for form controls. The order of rule blocks is
  * important as there are some rules with equal specificity that rely on order
  * as a tiebreaker. These are marked with OVERRIDE. */
 
 /* Default state **************************************************************/
 
@@ skipping 266 matching lines @@
  *
  *   <div class="checkbox">
  *     <label>
  *       <input type="checkbox"></input>
  *       <span>
  *     </label>
  *   </div>
  */
 
 :-webkit-any(.checkbox, .radio) label {
+  -webkit-user-select: none;
   /* Don't expand horizontally: <http://crbug.com/112091>. */
   align-items: center;
   display: inline-flex;
   padding-bottom: 7px;
   padding-top: 7px;
 }
 
 :-webkit-any(.checkbox, .radio) label input {
   flex-shrink: 0;
 }
 
 :-webkit-any(.checkbox, .radio) label input ~ span {
   -webkit-margin-start: 0.6em;
-  -webkit-user-select: none;
   /* Make sure long spans wrap at the same horizontal position they start. */
   display: block;
 }
 
 :-webkit-any(.checkbox, .radio) label:hover {
   color: black;
 }
 
 label > input:disabled:-webkit-any([type='checkbox'], [type='radio']) ~ span {
   color: #999;
 }
 
 extensionview {
   display: inline-block;
   height: 300px;
   width: 300px;
-}
+}