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

Merges [28219] to the 3.9 branch.

fixes #27936.

Built from https://develop.svn.wordpress.org/branches/3.9@28225


git-svn-id: http://core.svn.wordpress.org/branches/3.9@28053 1a063a9b-81f0-0310-95a4-ce76da25c4cd