The default sandbox config for tests built in chromium doesn't allow
access to the GPU or display. Switch to the gfx_tests sandbox config
to allow the tests to run.

Bug: angleproject:4350
Change-Id: Ib80db196ef55df694369cfc7a3cf18327ce7c952
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2023908
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>