Trace Tests: Use xvfb consistently on Linux. Previously there were two places where we missed the xvfb script. This was causing the replay to fail. Bug: angleproject:6085 Change-Id: I833916fa0cdacc163ec2bdd08831249807f319c5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2979353 Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>