[heycam/webidl] Is a subclass still a platform object? (#540)

Related a bit to #533.

I'm wondering if a subclass of, .e.g., `EventTarget`, would still be considered a platform object. Otherwise we need to expand the definition of "relevant global object" and potentially other such concepts.

And even then we might need to clarify things here, e.g.,
```html
<iframe></iframe>
...<script>
class X extends frames[0].EventTarget {};
</script>
```

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