Twenty Fourteen: Use correct check for the front page in Featured_Content::pre_get_posts().

props kwight.
fixes #29550.
Built from https://develop.svn.wordpress.org/trunk@29719


git-svn-id: http://core.svn.wordpress.org/trunk@29493 1a063a9b-81f0-0310-95a4-ce76da25c4cd