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This is a reland of https://crrev.com/c/2161689, which was
reverted in https://crrev.com/c/2163254 due to breakages on
the branch builders. The prerequisite changes have been
cherry-picked to the currently active branches (m81 and m83),
so this should be safe to reland.

We've already shown this to work on Mac10.15 Tests on the FYI
waterfall, so let's do this for the rest of the Mac triggered
testers on the main waterfall as well. This will allow us to
free up some Mac resources to do more important tasks (e.g.
compile, run tests). It also makes these builders builderless,
increasing our flexibility and improving our utilization.

This CL also introduces a new ci.triggered_tester starlark
function to enable easily specifying builders as "we don't care
what OS you use", and to enable easily turning all of them into
"thin testers" if/when we go down that path.

Bug: 1061582
Change-Id: I31b80f76784cc909633056ff9c71e03ebaf31504
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2170416
Commit-Queue: Aaron Gable <agable@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Auto-Submit: Aaron Gable <agable@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763416}