Remove hard-coded font families in WebUI.

chrome/browser/resources/supervised_user_block_interstitial.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. */
 body {
   background-color: rgb(230, 230, 230);
-  font-family: Helvetica, Arial, sans-serif;
-  font-size: 10pt;
   margin: 50px 40px 20px 40px;
   text-align: center;
 }
 
 #main-frame-blocked {
   margin: auto;
   max-width: 540px;
   min-width: 200px;
 }
 
@@ skipping 79 matching lines @@
   }
 
   #request-access-button {
     order: 1;
   }
 
   button {
     width: 100%;
   }
 }