Fix the incorrect font used by buttons in ChromeOS.

ui/webui/resources/css/text_defaults.css

 /* Copyright 2014 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.css">
  *
  * 2. via the webui::AppendWebUICSSTextDefaults() method to directly append it
  * to an HTML string.
  * Otherwise its $placeholders won't be expanded. */
 
 html {
   direction: $1;
 }
 
 body {
   font-family: $2;
   font-size: $3;
 }
+
+button {
+  font-family: $2;
+}