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This CL encapsulates the lacros test runner with either xvfb.py or
test_env.py to be consistent with other Desktop platforms.

Bug: 1104318
Change-Id: I00980225aff8856e5747d15f7e57814fb2e4043c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303169Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789214}