[permissions] Add "nfc" to enum PermissionName (#47)

In order to integrate the Permissions API with the [Web NFC API](http://w3c.github.io/web-nfc/index.html) spec, could you please include a new ```"nfc"``` entry to the permission name list.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/permissions/issues/47