bug#43071: Enable WebKit sandboxing

[Paul Eggert]

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.

0001-Use-WebKit-sandboxing.patch
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