For Lacros, xvfb is only used in Linux-Lacros, but not on-device
config, so this CL fixes the problem.

Bug: chromium:1195415
Test: built Chromium and Angle from source
Change-Id: Ia9adf706716c424b14a395189755932e445b7050
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2802854Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>