Currently the API in Chrome just returns whether a top-level window can prompt for/use permission. This is useful in some specific circumstances (e.g. for notification requests that happen in popups). But it's more consistent to return the result for the current context. 

Action items:
1) Decide whether to make an explicit exception for workers/worklets/SWs; if not, then update them to always return "denied" instead of "prompt" for all permissions that require prompting.
2) Update Permissions API spec to clarify this.
3) Update Notifications spec to refer to Permissions API spec for getting permission state.
4) Update Notification::permission & NotificationPermissionContext::DecidePermission to return "denied" instead of "prompt" in iframes (ditto for any similar permissions?).
5) Gather developer feedback on whether we need a new API to predict permission state in new popups.