The object Element.prototype[Symbol.unscopables] itself has a prototype but shouldn't

[pshaughn]

WPT WebIDL/emcascript-binding/interface-prototype-object.html tests this and fails; other browsers pass.

[pshaughn]

https://heycam.github.io/webidl/#interface-prototype-object 8.1 tells us to create this object with a null prototype. interface.rs create_unscopable_object calls JS_NewPlainObject, which is the wrong constructor: it's creating a "plain" object in the sense that {} is a plain object, having the default Object.prototype rather than the unusual special case of a completely absent prototype. I'll see if JS_NewObject(*cx, ptr::null()), as seen in create_callback_interface_object, has the desired result.

Update: No, it did not.