Remove hard-coded font families in WebUI.

chrome/browser/resources/ntp4/apps_page.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. */
 
 .app {
   outline: none;
   position: absolute;
   text-align: center;
 }
 
@@ skipping 89 matching lines @@
   visibility: hidden;
 }
 
 #app-launcher-promo {
   background-color: white;
   border: 1px solid lightgray;
   border-bottom-width: 3px;
   border-radius: 2px;
   border-top-width: 2px;
   bottom: 90px;
-  font-family: Arial, Helvetica, sans-serif;
   height: 120px;
   left: 50%;
   margin-left: -300px;
   position: fixed;
   width: 600px;
 }
 
 #app-launcher-promo > .close-button {
   position: absolute;
   right: 10px;
@@ skipping 33 matching lines @@
   background-image: -webkit-linear-gradient(
       top, rgb(77, 144, 254), rgb(53, 122, 232));
   border: 1px solid rgb(47, 91, 183);
 }
 
 #app-launcher-promo > img {
   bottom: 0;
   position: absolute;
   right: 30px;
 }