[Spec] Powerful API Restrictions in fenced frame

[sisidovski]

Powerful APIs are restricted in fenced frames by some security mechanism like permission policies or sandbox. However, some other APIs that can be communication channels between the embedder and the frame are currently restricted by per-API error handling. We need to update the spec to be consistent with the current implementation.

Target APIs:

• Background Fetch
• Background Sync
• Notification
• Push Messaging
• Payments (Payment Handler)
• Content Index
• Badging