About: Add a fallback `-webkit-calc()` to `about.css`

We forgot to run `grunt precommit` before [35834] was committed.

This change doesn't need to be backported to the 4.4 branch, as it only affects Chrome < 26, it's really just to clean up the behaviour of `grunt precommit`.

See #34663.



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git-svn-id: https://core.svn.wordpress.org/trunk@35826 1a063a9b-81f0-0310-95a4-ce76da25c4cd