Dimensions: Reverse a check to avoid potential reflows

Fixes #14979
Closes gh-1560
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.

Fixes #15016