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.

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git-svn-id: http://core.svn.wordpress.org/branches/4.0@30262 1a063a9b-81f0-0310-95a4-ce76da25c4cd