Eliminate the 1px padding adjustment on active buttons because it creates an un-clickable dead zone 1px high and spanning the entire width of the button.

Merges half of [26993] to the 3.8 branch.

props johnbillion.
see #26700.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@26998 602fd350-edb4-49c9-b593-d223f7449a82