clients.get() should not resolve before sandboxing determines the origin of the client

I think this is an oversight in the spec.

clients.get(id) does:
"Wait for either client’s execution ready flag to be set or for client’s discarded flag to be set."
https://w3c.github.io/ServiceWorker/#clients-get

The execution ready flag seems set at:
"7. Set the active document of browsingContext to document."
https://html.spec.whatwg.org/multipage/browsers.html#creating-a-new-browsing-context

Which later on does:
"11. Implement the sandboxing for document."

Probably we must implement the sandboxing for document first, and if the document has a unique origin we "Run the environment discarding steps for reservedEnvironment." like a cross-origin redirect. Then we can set execution ready.

Chrome currently resolves to a WindowClient. Firefox resolves with undefined. I think the spec intent is undefined.

cc @annevk @wanderview

Start with the clients.get(id) algorithm in the ServiceWorker specification and the linked HTML steps for creating a browsing context. Compare when the execution ready flag is set with when document sandboxing and environment discarding occur. Done means the specification clearly defines the result for a document with a unique origin.