bug#44204: bug#43071: Enable WebKit sandboxing

[Paul Eggert]

No further comment, so I installed the WebKit sandboxing patch into the emacs-27 
branch on Savannah 
<https://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-27&id=71661b287297f328c2c5ad67e180a760f80850cb>. 
After the next merge from emacs-27 to master, this patch should appear in the 
master branch. I am closing Bug#43071.

While testing the fix, I ran into a problem with xwidgets and filed Bug#44204 
"Emacs --with-xwidgets complains under Ubuntu 20.04" which you can see here:

https://bugs.gnu.org/44204

Has anyone had luck running Emacs --with-xwidgets under Ubuntu 20.04 or later? 
I'll cc. this to 44204@debbugs.gnu.org to try to move that part of the 
discussion there.