Named properties object: enumerate supported properties in [[OwnPropertyKeys]]

[shvaikalesh]

Currently, Gecko implements proposed [[OwnPropertyKeys]] semantics, but Blink and WebKit do not.

This PR feels like an expected behavior and aligns named properties object with legacy platform objects on enumerating supported property names.

Since only Window has named properties object, and it also has a LegacyUnenumerableNamedProperties extended attribute, this change has no performance impact on for (var k in window) {}.

If this is feasible to implement in Blink, I will contribute extensive WPT coverage and author a patch for WebKit. Once #963 and [[OwnPropertyKeys]] discrepancies are resolved, WindowProperties will be 100% compatible across all implementations!

[annevk]

How would for (var k in window) {} end up invoking this, exactly? Wouldn't that end up invoking https://html.spec.whatwg.org/#windowproxy-ownpropertykeys?

[shvaikalesh]

@annevk for/in indeed calls WindowProxy's [[OwnPropertyKeys]] first, but then goes up the prototype chain 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]].

[annevk]

Thank you! (I sometimes wonder if we should import more of this Window-specific logic into HTML, but that's also quite an ordeal.)

[shvaikalesh]

I've decided not to pursue this change: there is no clear benefit / use case to justify adding the method into 2 implementations, while other methods of named properties object remain unchanged.

Instead, to ensure interop, web-platform-tests/wpt#27970 adds a test that named properties objects has ordinary [[OwnPropertyKeys]] method.