Formalizing our permissions model using the Permissions API

I tried to think a bit about how I would formalize the permissions model
we have designed for WebRTC in terms of the permissions API's
permissions model.

After a while, it seemed to make sense, and I'd like to share it to see
if it makes sense to others.

The Google doc is here:

https://docs.google.com/document/d/13c4hTlm2XgVYpxfGL1a8fcvI1CAUdIgd662DfElk_ow/edit#heading=h.fb7kn49jp9ff

Anyone with the link should be able to comment, if you want to comment
off-list.

The content is attached as PDF, which may be more accessible to some
(and is certainly more archivable).

Questions to ask:

- Is this something worth doing?
- If yes: Is this something that should be part of our core docs, a
separate doc, part of the permissions doc, or in some "other" category?
- Where did I go wrong?

Comments welcome!

Harald