Issue #1970 - Part 5: Move combobox/select padding to the select rule.

This doesn't change behavior by default but allows authors to remove the padding
previously enforced to align with focus ring padding, if they wish to.

layout/reftests/forms/select/vertical-centering.html

@@ -7,11 +7,10 @@
   select {
     -moz-appearance: none;
     border: none;
-    font-family: Ahem;
-    font-size: 20px;
-    padding: 0;
+    font: 20px/1 Ahem;
     box-sizing: content-box;
     height: 40px;
+    padding: 0;
   }
 </style>
 <select>