Media Frames: Set the box-sizing CSS property to border-box for checkboxes and radio buttons.

Merges [29727] to the 4.0 branch.

props mlteal.
fixes #29493.


git-svn-id: https://develop.svn.wordpress.org/branches/4.0@30262 602fd350-edb4-49c9-b593-d223f7449a82