Drop the -webkit prefix from user-select CSS rule in ui/.

ui/webui/resources/css/expandable_bubble.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. */
 
 .expandable-bubble {
   -webkit-border-image: url(chrome://theme/IDR_APP_NOTIFICATION_SMALL_BUBBLE)
                         5 5 7 6 stretch;
   -webkit-box-sizing: border-box;
-  -webkit-user-select: none;
   border-width: 5px 5px 7px 6px;
   color: #444;
   cursor: pointer;
   display: inline-block;
   font-size: 12px;
   position: absolute;
+  user-select: none;
   z-index: 1;
 }
 
 .expandable-bubble::after {
   bottom: -1px;
   content: url(chrome://theme/IDR_APP_NOTIFICATION_NUB);
   display: block;
   height: 7px;
   position: absolute;
   right: 5px;  /* TODO(finnur): Need to handle RTL properly. */
@@ skipping 55 matching lines @@
   background-image: -webkit-image-set(
       url(../../../resources/default_100_percent/close_2_hover.png) 1x,
       url(../../../resources/default_200_percent/close_2_hover.png) 2x);
 }
 
 .expandable-bubble-close:active {
   background-image: -webkit-image-set(
       url(../../../resources/default_100_percent/close_2_pressed.png) 1x,
       url(../../../resources/default_200_percent/close_2_pressed.png) 2x);
 }