Header Images: Add suggested dimensions to the media workflow.

props ehg, gcorne.
see #21785.


git-svn-id: https://develop.svn.wordpress.org/trunk@28030 602fd350-edb4-49c9-b593-d223f7449a82
Header Images: Add suggested dimensions to the media workflow.

This updates [28030] to the latest patch.

props gcorne.
see #21785.


git-svn-id: https://develop.svn.wordpress.org/trunk@28031 602fd350-edb4-49c9-b593-d223f7449a82