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 def4841c)

Fixes #15016
Closes gh-1563

test/unit/css.js

@@ -754,14 +754,7 @@ test("widows & orphans #8936", function () {
 	var $p = jQuery("<p>").appendTo("#qunit-fixture");
 
 	if ( "widows" in $p[0].style ) {
-		expect(4);
-		$p.css({
-			"widows": 0,
-			"orphans": 0
-		});
-
-		equal( $p.css("widows") || jQuery.style( $p[0], "widows" ), 0, "widows correctly start with value 0");
-		equal( $p.css("orphans") || jQuery.style( $p[0], "orphans" ), 0, "orphans correctly start with value 0");
+		expect(2);
 
 		$p.css({
 			"widows": 3,