Update "creating a feature policy" to use embedder

[domenic]

HTML calls "creating a feature policy" in https://html.spec.whatwg.org/#creating-a-new-browsing-context. However, the "creating a feature policy" algorithm asks whether it's operating on a nested browsing context. And, at the time that call is made, the browsing context is never a nested BC, because we're still in the process of creating it; we haven't set up the parent relationships.

whatwg/html#5583 we changed things a bit to make this setup more resilient. In particular whatwg/html@3511b20 fixed the analogous problem for the sandboxing flags.

Concretely, it looks like the algorithm should be updated to accept null or an element embedder, instead of a browsing context. And then it should pass that through to https://w3c.github.io/webappsec-feature-policy/#define-inherited-policy, which can just check if embedder is null or not, and if it is not, use embedder directly.

Then we'd update the HTML call site and remove the red box.