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

chrome/browser/resources/sync_setup_overlay.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. */
 
 /* TODO(jhawkins): Organize these by page. */
 
 #sync-setup-overlay * h4 {
   margin: 5px 0;
 }
 
@@ skipping 190 matching lines @@
 #password-row {
   margin-top: 2px;
 }
 
 #action-area {
   margin-top: 2px;
 }
 
 #sync-setup-configure {
   background: white;
-}
-
-html[os='mac'] #sync-setup-configure {
+<if expr="is_macosx">
   line-height: 1.5em;
+</if>
 }
 
 #choose-data-types-form {
   -webkit-user-select: none;
 }
 
 #chooseDataTypesRadio {
   vertical-align: top;
 }
 
@@ skipping 252 matching lines @@
 }
 
 .signin-box .help-link:visited {
   color: #fff !important;
 }
 
 .signin-box .help-link:hover {
   color: #fff !important;
   opacity: .7;
 }