Return container shadows to 1px blur as they were pre- r26293. This is a...

Return container shadows to 1px blur as they were pre- r26293. This is a better match for the 1px shadow we use on buttons and screen options. See #25858.
Built from https://develop.svn.wordpress.org/trunk@26841


git-svn-id: https://core.svn.wordpress.org/trunk@26728 1a063a9b-81f0-0310-95a4-ce76da25c4cd