Re: [heycam/webidl] Named properties object: enumerate supported properties in [[OwnPropertyKeys]] (#964)

@annevk `for/in` indeed calls [`WindowProxy`'s `[[OwnPropertyKeys]]`](https://html.spec.whatwg.org/#windowproxy-ownpropertykeys) first, but then [goes up the prototype chain](https://tc39.es/ecma262/#sec-for-in-iterator-objects) for same-origin windows.

WebKit, and I believe other runtimes as well, accept additional parameter when implementing `[[OwnPropertyKeys]]` so non-enumerable property names (for `Object.keys` and `for/in`) can be rejected without calling `[[GetOwnProperty]]`. 

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