[webui] get rid of more html[os=] rules

chrome/browser/resources/shared/css/menu.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.
  */
 
 menu {
   -webkit-box-shadow: 0 2px 4px rgba(0, 0, 0, .50);
   background: white;
   color: black;
   cursor: default;
@@ skipping 32 matching lines @@
                                       rgba(0, 0, 0, .02) 96%);
   border: 0;
   height: 1px;
   margin: 8px 0;
 }
 
 menu > [disabled] {
   color: rgba(0, 0, 0, .3);
 }
 
-/* Text alpha doesn't work on Linux/GTK and ChromeOs/views.
- * TODO(estade): http://crbug.com/88688. */
-html[toolkit=gtk] menu > [disabled],
-html[os=chromeos] menu > [disabled] {
-  color: #969696;
-}
-
 menu > [hidden] {
   display: none;
 }
 
 menu > :not(hr)[selected] {
   background-color: #dce5fa;
 }
 
 menu > :not(hr)[selected]:active {
   background-color: #426dc9;
   color: #fff;
 }
 
 menu > [checked]:before {
   content: url("../images/checkbox_black.png");
   display: inline-block;
   height: 9px;
   margin: 0 5px;
   vertical-align: 50%;
   width: 9px;
 }
 
 menu > [checked] {
   -webkit-padding-start: 0;
 }
 
 menu > [selected][checked]:active:before {
   content: url("../images/checkbox_white.png");
 }