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

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 {
   outline: none;
   overflow: auto;
   display: block;
 }
 
 .tree-item > .tree-row {
   color: black;
   -webkit-user-select: none;
   border: 1px solid rgba(255,255,255,0); /* transparent white */
@@ skipping 139 matching lines @@
   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:not([os=mac]) .tree-item[editing] input {
-  outline: none;
-}
-
 html[dir=rtl] .tree-item[editing] input {
   margin: -2px -3px -2px -8px;
   padding: 1px 1px 1px 7px;
 }