Remove hard-coded font families in WebUI.

chrome/browser/resources/print_preview/previewarea/margin_control.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. */
 
 #preview-area .margin-control {
   -webkit-transition: opacity 150ms linear;
   position: absolute;
 }
 
 #preview-area .margin-control.invisible {
@@ skipping 38 matching lines @@
 #preview-area .margin-control-right .margin-control-line {
   height: 100%;
 }
 
 #preview-area .margin-control-textbox {
   background-color: #2a2a2a;
   border: 1px solid #888;
   box-sizing: border-box;
   color: white;
   cursor: auto;
-  font-family: arial;
   font-size: 0.8em;
   height: 25px;
   padding: 5px 0;
   position: absolute;
   text-align: center;
   width: 60px;
 }
 
 #preview-area .margin-control-textbox.invalid {
   background-color: rgb(193, 27, 23);
@@ skipping 11 matching lines @@
 
 #preview-area .margin-control-bottom .margin-control-textbox {
   bottom: 8px;
   right: 50%;
 }
 
 #preview-area .margin-control-left .margin-control-textbox {
   bottom: 50%;
   left: 8px;
 }