[Spec] Specify way to exempt URLs from network revocation

[blu25]

For web platform tests, we still need to be able to communicate with a fenced frame after network revocation in order to test that certain functionalities stop working. We currently do this with an Chromium-specific window.internals API that allows specified URLs to be exempt from network revocation. To launch network revocation with working tests to the wider community, we will need to have a web driver API that does the same thing. This will require a change to the WPT web driver + an RFC opened for it.

[domfarolino]

Can you describe why this is important / why we filed this? It isn't for web-exposed behavior right? Testing-only? (A brief description of how this came about / what PR prompted this would be good).