Fix JSHint errors in custom-header.js and custom-background.js.

props jorbin.
fixes #25915, #25916.

Built from https://develop.svn.wordpress.org/trunk@26158


git-svn-id: http://core.svn.wordpress.org/trunk@26069 1a063a9b-81f0-0310-95a4-ce76da25c4cd