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Fixes swarming-related mixins not working on CrOS builders due to
generate_buildbot_json.py using the wrong dictionary in one spot.

Also switches several CrOS builders to use mixins instead of hard-coded
swarming dimensions since it now works.

Bug: 1080809
Change-Id: Idcde77f97c16a9039d719efd7d2fc750dbede885
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2200079
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768501}