cq: start using LUCI downstream testers as an experiment.
cq: start using LUCI downstream testers as an experiment.

R=hinoka

Bug: 830992
Change-Id: Ibb0a269c10d2939505e4286e3278efb650d82636
Reviewed-on: https://chromium-review.googlesource.com/1024869
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools