[whatwg/dom] Should adopting change an element's prototype (#731)

```js
document.body.append(
  iframe.contentDocument.createElement('p'),
);
```

Will the paragraph have a prototype from the parent or the iframe?

Test: https://static-misc.glitch.me/custom-element-adoption-test/ (see console).

Chrome/Safari leave the prototype as it is.

Firefox/Edge change the prototype to one from the parent page.

Chrome/Safari/Edge change the prototype if the element is a custom element. I assume this is standard, but I haven't yet found it in the spec.

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