Header Images: Add suggested dimensions to the media workflow.

props ehg, gcorne.
see #21785.

Built from https://develop.svn.wordpress.org/trunk@28030


git-svn-id: http://core.svn.wordpress.org/trunk@27860 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Header Images: Add suggested dimensions to the media workflow.

This updates [28030] to the latest patch.

props gcorne.
see #21785.

Built from https://develop.svn.wordpress.org/trunk@28031


git-svn-id: http://core.svn.wordpress.org/trunk@27861 1a063a9b-81f0-0310-95a4-ce76da25c4cd