Do not allow for forcing classic scrollbars

[sadym-chromium]

In some cases it is non-trivial and meaningless to try to emulate classic scrollbar. E.g. on mobile device, while forcing overlay scrollbar instead of the classic one is very common use-case. This PR restricts the emulation to only force using overlay scrollbars.

---

Preview | Diff

[gsnedders]

So on macOS, where overlay scrollbars are (more-or-less) the default, we now have emulation.SetScrollbarTypeOverrideParameters.scrollbarType with the values: "default" (overlay), "overlay", or null (the default, so overlay).

This fundamentally does not work for WPT where large numbers of tests rely on classic scrollbars.

This is far from the only case with emulation where we're going to run into "the endpoint node doesn't support this", and in almost every case we should just be returning an error where it isn't supported.

Endpoints which only support "overlay" scrollbars should return an error when you try and set it; endpoints which only support "classic" scrollbars should return an error when you try and set it.

[sadym-chromium]

So on macOS, where overlay scrollbars are (more-or-less) the default, we now have emulation.SetScrollbarTypeOverrideParameters.scrollbarType with the values: "default" (overlay), "overlay", or null (the default, so overlay).

This fundamentally does not work for WPT where large numbers of tests rely on classic scrollbars.

This is far from the only case with emulation where we're going to run into "the endpoint node doesn't support this", and in almost every case we should just be returning an error where it isn't supported.

Endpoints which only support "overlay" scrollbars should return an error when you try and set it; endpoints which only support "classic" scrollbars should return an error when you try and set it.

Hi @gsnedders! Thanks for your comment. I see the motivation, but I'm not sure if I understood your proposal. Which of the following options seems reasonable for you?

1. (current) classic / overlay / null
2. default / overlay / null
3. default / classic / overlay / null