[spec-reviews] W3C Push API - data payloads (#98)

The W3C Push API has been updated by @martinthomson to allow developers to include a payload with the push messages they send. Such a payload must be encrypted, because the push service effectively is an untrusted intermediary.

The W3C bits are defined in https://w3c.github.io/push-api/
```
PushSubscription.getKey(type);  // "p256dh" or "auth", returning an ArrayBuffer
PushSubscription serializer;  // includes the keys, base64url encoded, in the JSON
PushMessageData
PushEvent.data
```

The client-side JavaScript will normally not be exposed to the encryption requirement - this is done by the application server. The data available in `PushEvent.data` will have been **decrypted** already.

The encryption routines themselves are defined in the following two IETF drafts:

- https://tools.ietf.org/html/draft-ietf-webpush-encryption
- https://tools.ietf.org/html/draft-thomson-http-encryption

Chrome and Firefox are both eager to ship.

/cc @mvano

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Reply to this email directly or view it on GitHub:
https://github.com/w3ctag/spec-reviews/issues/98