Enable sandboxed process for XR browser tests

Enables the use of the sandboxed process for the XR browser tests. This
is achieved by running the tests through a wrapper script that sets the
correct ACLs on the output directory before running the tests.

Bug: 949619
Change-Id: If583d50234f8d763da23cd70ab65b898a4164492
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1555499
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649623}