style: Restore the order of Scrollbarbutton appearance values.
style: Restore the order of Scrollbarbutton appearance values.

The patch at bug 1478391 comment 6 changed the way the math in Scrollbarbutton*
worked, which pretty surely caused this.

Restore the original order and math to be the same as before bug 1478391.

Bug: 1479216
Reviewed-by: xidorn
MozReview-Commit-ID: CK3iOqeX2NW

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