presubmit_unittest.py: use LUCI for the optional GPU tests on Windows
presubmit_unittest.py: use LUCI for the optional GPU tests on Windows

BUG=chromium:815092

Change-Id: Iecbbdf41a00a637e07dfb137d7956f110138b5f6
Reviewed-on: https://chromium-review.googlesource.com/1008302
Reviewed-by: Aaron Gable <bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org>
Commit-Queue: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools