This makes trigger.py use the same code paths as the bots
when they trigger standalone ANGLE tests.

Fixes launching tests via trigger.py on Linux because of
using the 'windowed_test_launcher' method that used xvfb.

Bug: angleproject:5114
Change-Id: I073631270b5b69aac8f3f0f6378bd71f951ce8e6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2464425
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>