Avoid unintentional overriding of .spinner styles for HiDPI displays.

props mauryaratan for initial patch.
fixes #27269.
Built from https://develop.svn.wordpress.org/trunk@27504


git-svn-id: http://core.svn.wordpress.org/trunk@27347 1a063a9b-81f0-0310-95a4-ce76da25c4cd