Move the check for body.mobile in postbox.js to a variable as it's used in...

Move the check for body.mobile in postbox.js to a variable as it's used in more than one place, props georgestephanis, scribu, fixes #20561

git-svn-id: https://core.svn.wordpress.org/trunk@20677 1a063a9b-81f0-0310-95a4-ce76da25c4cd