More specific selectors for the 2x images take 2, don't specify...

More specific selectors for the 2x images take 2, don't specify background-size for the 1x background, see #20293

git-svn-id: https://core.svn.wordpress.org/trunk@20364 1a063a9b-81f0-0310-95a4-ce76da25c4cd