[mac] Eliminate drawing glitch on WebUI radio controls.

chrome/browser/resources/shared/css/checkbox.css

 /* Copyright (c) 2011 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.
  */
 
 input[type='checkbox'] {
   -webkit-box-shadow: inset 0 1px 2px white,
                       0 1px 2px rgba(0, 0, 0, .2);
   -webkit-appearance: none;
   -webkit-margin-start: 0;
@@ skipping 45 matching lines @@
   -webkit-appearance: none;
   -webkit-margin-start: 0;
   -webkit-margin-end: 3px;
   -webkit-transition: border 500ms;
   background: -webkit-linear-gradient(#fafafa, #dcdcdc);
   border-radius: 100%;
   border: 1px solid #a0a0a0;
   display: inline-block;
   height: 15px;
   margin-bottom: 0;
-  margin-top: 0;
   position: relative;
   top: 3px;
   vertical-align: baseline;
   width: 15px;
 }
 
 input[type='radio']:disabled {
   opacity: .75;
 }
 
@@ skipping 51 matching lines @@
 
 label:hover > input[type=checkbox]:disabled ~ span,
 label:hover > input[type=radio]:disabled ~ span {
   color: #888;
 }
 
 label:hover > input[type=checkbox]:not(:disabled) ~ span,
 label:hover > input[type=radio]:not(:disabled) ~ span {
   color: #222;
 }