If the `XAUTHORITY` env var is unset, libXau will fall back to
`$HOME/.Xauthority`, but our content sandbox policy didn't handle that
case when it needs to allow access to that file; this patch corrects
that oversight.
This broke WebGL as of bug 1635451, because we no longer eagerly connect
to the X server before sandbox startup, only as needed for WebGL.
Usually the `XAUTHORITY` env var is set even if the file is in its
default location, but some environments (including but not limited to
the Linux VMs on Chrome OS) do not set it.
Differential Revision: https://phabricator.services.mozilla.com/D127984
cbde13526d