Move getSelection to Document and ShadowRoot; Mark ShadowRoot.getSelection as non-standard.

[Elchi3]

Fixes #7891
Spec: https://w3c.github.io/selection-api/#extensions-to-document-interface

Delete getSelection from DocumentOrShadowRoot files and add it to Document. On Window, it already exists.

MDN content PR: mdn/content#3037

[foolip]

Chromium does have ShadowRoot#getSelection because it's in the DocumentOrShadowRoot mixin here:
https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/dom/document_or_shadow_root.idl;l=12?q=shadow_root.idl

Also confirmed with https://mdn-bcd-collector.appspot.com/tests/api/ShadowRoot/getSelection

However, that entry isn't on the same footing as the one on Document. It should be marked as non-standard and supported only in Chromium browsers, presumably.

[foolip]

w3c/selection-api#114 is the spec issue for this situation.

[foolip]

I've sent #9434. With that merged it should be a simple matter of moving the entries around.

[Elchi3]

Thanks @foolip! Now this PR looks a bit more silly than before, but I guess it is still correct given the chromium implementation could remain non-standard.

[foolip]

Shouldn't the other one also be moved and have its mdn_url either updated or removed?

[Elchi3]

Moved It and removed the mdn_url.

We will see long-term if all this moving around makes sense. It doesn't change what BCD exposes after all.

[foolip]

I'll leave merging to you, since the description needs updating.