There's no reason to make fetch of about:blank same-origin for loads other than navigations and `<object>`

[bzbarsky]

I don't think it's required for web compat. Gecko only does this for navigations and <object> (things that can end up with a browsing context). Specifically, about:blank stylesheets are cross-origin in Gecko, and you can't XHR to about:blank in either Gecko or Blink (but can in WebKit, apparently).

[annevk]

@youennf what do you think?

Navigate is already always same-origin. <object> would require some special casing but that seems fine.

If we changed this we'd automatically fix #385 since all about URL fetching would just be cross-origin (unless it's the result of a navigation).

[youennf]

I don't think XHR/fetch is working for about:blank in WebKit.
Sounds good to make consistent all about URLs.

[annevk]

Thanks.