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

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

Index: chrome/browser/resources/shared/css/checkbox.css
===================================================================
--- chrome/browser/resources/shared/css/checkbox.css	(revision 114042)
+++ chrome/browser/resources/shared/css/checkbox.css	(working copy)
@@ -13,8 +13,6 @@
   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;
@@ -43,15 +41,13 @@
 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);