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

chrome/browser/resources/bookmark_manager/css/bmm.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.
  */
 html, body {
   margin: 0;
   width: 100%;
   height: 100%;
   cursor: default;
   overflow: hidden;
@@ skipping 114 matching lines @@
   -webkit-margin-start: -4px;
   -webkit-margin-end: 4px;
   max-width: 40%;
   padding: 0;
   -webkit-padding-start: 1px;
   -webkit-padding-end: 3px;
   text-decoration: none;
 }
 
 .tree-item [editing] input {
+  -webkit-margin-start: -3px;
+  -webkit-padding-end: 7px;
   margin: 0;
   padding: 0;
-  -webkit-margin-start: -3px;
-  -webkit-padding-end: 7px;
+<if expr="is_macosx">
+  padding-top: 1px;
+</if>
 }
 
-html[os=mac] .tree-label [editing] input {
-  padding-top: 1px;
-}
-
-html:not([os=mac]) list .label input,
-html:not([os=mac]) list .url input {
+<if expr="is_macosx">
+list .label input,
+list .url input {
   outline: none;
 }
+</if>
 
 list > [editing] {
   overflow: visible;
 }
 
 list [editing] .label,
 list [editing] .url,
 list [editing] > * {
   overflow: visible;
   display: inline;
 }
 
 list [editing] .url {
   -webkit-padding-start: 10px;
 }
 
 /* end editing */
 
 html[dir=rtl] list > .folder > .label {
   background-image: url("../../../../../ui/resources/folder_closed_rtl.png");
 }
 
-html[os=mac] list > .folder > .label {
+<if expr="is_macosx">
+list > .folder > .label,
+.tree-label,
+.tree-row[may-have-children] > .tree-label,
+.tree-item[expanded] > .tree-row > .tree-label {
   background-image: url("../../../../app/theme/bookmark_bar_folder_mac.png");
 }
-
-html[os=mac] .tree-label,
-html[os=mac] .tree-row[may-have-children] > .tree-label {
-  background-image: url("../../../../app/theme/bookmark_bar_folder_mac.png");
-}
-
-html[os=mac] .tree-item[expanded] > .tree-row > .tree-label {
-  background-image: url("../../../../app/theme/bookmark_bar_folder_mac.png");
-}
+</if>
 
 .main {
   position: absolute;
   display: -webkit-box;
   top: 79px;
   left: 0;
   right: 0;
   bottom: 0;
 }
 
@@ skipping 21 matching lines @@
 #list {
   -webkit-box-flex: 1;
   box-sizing: border-box;
   padding: 5px;
 }
 
 .splitter {
   width: 5px;
   background-color: #ebeff9;
   cursor: e-resize;
-}
-
-html[os=mac] .splitter {
+<if expr="is_macosx">
   cursor: col-resize;
+</if>
 }
 
 .logo {
   -webkit-appearance: none;
   border: 0;
   background: url("../images/bookmarks_section_32.png") no-repeat 50% 50%;
   width: 32px;
   height: 32px;
   cursor: pointer;
   margin: 10px;
@@ skipping 67 matching lines @@
   font: inherit;
   padding: 0;
   background: -webkit-canvas(drop-down-arrow) 100% 50% no-repeat;
   -webkit-padding-end: 11px;
   -webkit-margin-start: 10px;
 }
 
 html[dir=rtl] .summary button {
   background-position: 0 50%;
 }