Reintroduce fix for WebKit audio/video control bug

This fix was first introduced to deal with Chrome < 13 destroying the
appearance of native `audio` and `video` button controls when
`-webkit-appearance` is set on `input[type="button"]`. See #20

Android 4.0.* seems to use a version of WebKit that contains this bug.
See: https://github.com/h5bp/mobile-boilerplate/issues/121

...so the fix needs to be reintroduced.