[Template Expressions] fix for incognito getting wrong font style

ui/webui/resources/css/text_defaults_md.css

 /* Copyright 2015 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. */
 
 /* This file is dynamically processed by a C++ data source handler to fill in
  * some per-platform/locale styles that dramatically alter the page. This is
  * done to reduce flicker, as JS may not run before the page is rendered.
  *
  * There are two ways to include this stylesheet:
  * 1. via its chrome://resources/ URL in HTML, i.e.:
  *
  *   <link rel="stylesheet" href="chrome://resources/css/text_defaults_md.css">
  *
  * 2. via the webui::AppendWebUICSSTextDefaultsMd() method to directly append it
  * to an HTML string.
  * Otherwise its $placeholders won't be expanded. */
 
 @import url(chrome://resources/css/roboto.css);
 
 html {
-  direction: ${textDirection};
+  direction: $i18n{textDirection};
 }
 
 body {
-  font-family: Roboto, ${fontFamily};
-  font-size: ${fontSize};
+  font-family: Roboto, $i18n{fontFamily};
+  font-size: $i18n{fontSize};
 }
 
 button {
-  font-family: Roboto, ${fontFamily};
+  font-family: Roboto, $i18n{fontFamily};
 }