Autoprefixer for [30857].

Merges [30884] to the 4.1 branch.

See #30435.

Built from https://develop.svn.wordpress.org/branches/4.1@30902


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30892 1a063a9b-81f0-0310-95a4-ce76da25c4cd