ResizeObserver not present on the Window interface

[guillaumebrunerie]

The ResizeObserver class is only available as a global declaration and not on the Window interface. I think it should also be available on the Window interface for at least two reasons:

• The specification (https://drafts.csswg.org/resize-observer/#idl-index) says [Exposed=(Window)], which I’m guessing means that it is supposed to be on the Window interface (I’m actually surprised, isn’t the specification supposed to be automatically translated to Typescript declarations?)
• For applications with several windows, it behaves differently depending on the window from which it is taken (see https://codepen.io/Latcarf/pen/RwdyLXr?editors=0010). So it would make a lot of sense to be able to do the following, which currently doesn’t typecheck as popup is of type Window.

  const popup = window.open(...)!;
  new popup.ResizeObserver(...);

[turansky]

says [Exposed=(Window)]

Window in this case is environment name, like DedicatedWorker or SecureContext (no such interfaces)

Also:

1. Global ResizeObserver is fine
2. There is no support for nested interfaces/types in TS
3. It could't be implemented without duplication