Remove hard-coded font families in WebUI.

chrome/browser/resources/chromeos/keyboard_overlay.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.
  */
 
 body {
   -webkit-user-select: none;
   background: #fff;
   margin: 0;
   overflow: hidden;
   padding: 0;
 }
 
 .keyboard-overlay-keyboard {
   background: -webkit-linear-gradient(#484848, #252525) no-repeat;
   background-color: #252525;
   border-radius: 6px;
-  font-family: 'Noto Sans UI', ui-sans;
 }
 
 .keyboard-overlay-instructions {
   -webkit-box-orient: vertical;
   background: -webkit-linear-gradient(rgb(51, 76, 126), rgb(13, 23, 43));
   border: 2px solid rgb(87, 108, 207);
   border-radius: 5px;
   color: #fff;
   display: -webkit-box;
   position: absolute;
@@ skipping 150 matching lines @@
 
 .keyboard-overlay-key-text {
   -webkit-box-ordinal-group: 2;
   padding: 0 3px 2px;
   text-align: center;
 }
 
 .keyboard-overlay-key.is-shortcut .keyboard-overlay-key-text {
   opacity: 0.25;
 }