[permissions] Add 'background-sync' permission (#60)

Users should be able to disable [background sync](https://wicg.github.io/BackgroundSync/spec/). There is already [discussion](https://github.com/WICG/BackgroundSync/issues/122) going on about what to do if permission is blocked. In the meantime, it is desirable to expose this information on the Permissions API.

I suggest adding a `background-sync` element to PermissionName. I am happy to provide a PR with the change.

What do you think?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/permissions/issues/60