Fix the positioning of the checkmark in WebUI pages for all platforms.

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;
   -webkit-margin-end: 3px;
   background: -webkit-linear-gradient(#fafafa, #dcdcdc);
   border-radius: 3px;
   border: 1px solid #a0a0a0;
   display: inline-block;
-  font: inherit;  /* TODO(csilv): investigate why this is required on the
-                     Windows platform. http://crbug.com/105811 */
   height: 16px;
   margin-bottom: 0;
   margin-top: 0;
   position: relative;
   top: 3px;
   vertical-align: baseline;
   width: 16px;
 }
 
 input[type='checkbox']:disabled {
   opacity: .75;
 }
 
 input[type='checkbox']:not(:disabled):not(:active):hover {
   background: -webkit-linear-gradient(#fff, #e6e6e6);
   text-shadow: 0 1px 0 rgba(255, 255, 255, 1);
 }
 
 input[type='checkbox']:not(:disabled):active {
   -webkit-box-shadow: inset 0 1px 3px rgba(0, 0, 0, .2);
   background: -webkit-linear-gradient(#f0f0f0, #bebebe);
   border: 1px solid #808080;
   text-shadow: 0 1px 0 rgba(255, 255, 255, .25);
 }
 
 input[type='checkbox']:checked::before {
   color: #808080;
   content: url('../images/checkmark.png');
+  font-size: 13px;  /* Explicitly set the font size so that the positioning
+                       of the checkmark is correct. */
   height: 16px;
   left: 2px;
   position: absolute;
 }
 
-html[os=mac] input[type='checkbox']:checked::before {
-  top: 2px;
-}
-
 input[type='radio'] {
   -webkit-box-shadow: inset 0 1px 2px white,
                       0 1px 2px rgba(0, 0, 0, .2);
   -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;
@@ skipping 64 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;
 }