[w3c/permissions] Flesh out request("push") (#111)

1. [`serviceWorkerRegistration.pushManager.subscribe()`](https://w3c.github.io/push-api/#widl-PushManager-subscribe-Promise-PushSubscription--PushSubscriptionOptionsInit-options) needs a `ServiceWorkerRegistration`. Should that be a required member of [`PushPermissionDescriptor`](https://w3c.github.io/permissions/#dictdef-pushpermissiondescriptor) or what?
2. [`subscribe()`](https://w3c.github.io/push-api/#widl-PushManager-subscribe-Promise-PushSubscription--PushSubscriptionOptionsInit-options) fails in at least 4 ways. We need to figure out which of these reject the `request()` promise, and which are communicated in other ways. Having the permission be denied is not exceptional for permissions.request(), so at least NotAllowedError needs to be a fulfillment with a particular state.
   1. SecurityError
   1. InvalidAccessError
   1. InvalidStateError
   1. NotAllowedError
   1. AbortError

@mounirlamouri @mvano @martinthomson (Martin, this doesn't presuppose the answer to #83, but the complexity of specifying request() could affect the decision there.)

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