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This tester runs three kinds of tests:
* chromium_mac_gtests, which are completely swarmed
* chromium_mac_rel_isolated_scripts, which are also swarmed
* chromium_mac_scripts, which are identical to chromium_linux_scripts,
  and which run on linux just fine

This means that the OS of the Mac10.15 Tests builder shouldn't
matter, all that matters is the OS of the hosts running its
triggered swarming tasks, which this CL doesn't change.

Assuming this goes well, I will also convert the other Mac testers
(10.10, 10.11, 10.12, 10.13, and 10.14, all on the main waterfall)
to also use Linux hosts. This will both free up some mac capacity
to use for compilers and testers, and make these builders
"builderless" which should help with capacity and scheduling.

Bug: 1061582
Change-Id: I307db015865288136e7fec0921942e051547be90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146053
Commit-Queue: Aaron Gable <agable@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Auto-Submit: Aaron Gable <agable@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758329}