[mac] Eliminate drawing glitch on WebUI radio controls.

BUG=105755
TEST=Verify that visual glitch is gone, see bug for details.
Review URL: http://codereview.chromium.org/8828011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113314 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -63,7 +63,6 @@ input[type='radio'] {
   display: inline-block;
   height: 15px;
   margin-bottom: 0;
-  margin-top: 0;
   position: relative;
   top: 3px;
   vertical-align: baseline;