Css: Use only positive integers as values for widows/orphans

According to http://www.w3.org/TR/CSS2/page.html#propdef-orphans, widows
and orphans can take only positive integer values. Together with a Chrome 34
regression this caused the test to fail.

(cherry-picked from def4841cfaf68e2ec156c554f8efa4aa64203f36)

Fixes #15016
Closes gh-1563