[permissions] Support for background sync (one-off and periodic) (#29)

The Background sync IDL is being updated: we are moving toward a design where periodic and one-off background sync would be independent. The UA may place these behind separate permissions.

https://github.com/jakearchibald/BackgroundSync/blob/mvp/idl.md (Review on-going)

Strawman:

```javascript
enum PermissionName {
    "geolocation",
    "notifications",
    "push",
    "midi",
    "backgroundsync"
};

dictionary BackgroundSyncPermissionDescriptor : PermissionDescriptor {
    enum BackgroundSyncType type;
};

enum BackgroundSyncType {
   "one-off",
   "periodic"
};
```

cc/ @jakearchibald @jkarlin @annevk

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/permissions/issues/29