[Spec] How fenced frames behaves with embedder policy?

[yoshisatoyanagisawa]

Fenced frames will be treated as top-level browsing contexts, and the existing spec says "If target is not a child browsing context, then return true." Therefore, no embedder policy would be applied if we literary understand the spec.

We might need to update the spec to tell how the policy should be applied for fenced frames.

[domfarolino]

This is done with the changes made in https://wicg.github.io/fenced-frame/#coop-coep, per #69.