bug#43071: Enable WebKit sandboxing

[Stefan Kangas]

Paul Eggert <eggert@cs.ucla.edu> writes:

> Qiantan Hong suggested that Emacs should enable sandboxing in WebKit, for all
> the usual security reasons. (Thanks, Qiantan!)
>
> Attached is a proposed patch to implement that suggestion; it's a bit fancier
> than what Qiantan originally proposed in
> <https://lists.gnu.org/r/emacs-devel/2020-08/msg00896.html> because it checks
> that WebKit 2.26 or later is in use, and it avoids a duplicate call to
> webkit_web_context_get_default. I'm cc'ing this to Qiantan and to other recent
> committers to xwidget.c, to get their opinions.

Thanks Qiantan Hong and Paul Eggert.

Is this important enough to warrant backporting to emacs-26?