Customizer: Allow to skip cropping header images if image width is smaller than or equal to theme width.

Add tests.

see #27936 for trunk.
Built from https://develop.svn.wordpress.org/trunk@28219


git-svn-id: http://core.svn.wordpress.org/trunk@28049 1a063a9b-81f0-0310-95a4-ce76da25c4cd