Trace Tests: Add xvfb command line option.
Trace Tests: Add xvfb command line option.

This will enable running the tests on Linux.

Bug: angleproject:5530
Change-Id: I7cd2155fac89d395a8bebacb2f1a44be0e7d3739
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2644723
Reviewed-by: Cody Northrop <94770d7fe1cce6ea0791fa9927c1107de4d2e37b@google.com>
Reviewed-by: Tim Van Patten <c7814369a8d3d5671c484acc7b3215f1bf4221be@google.com>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle