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In the case of migrating Chrome mac 10.9 bots to 10.13, make use of
the trigger tool
//testing/trigger_scripts/trigger_multiple_dimensions_unittest.py, to
support buider chromium.mac bot 'Mac10.9 Tests' running tests on
either 10.9 or 10.13 bots in a load-balancing way.

This CL:
1) adds add_common_test_properties in generate_buildbot_json.py which
adds the tigger script logic.
2) add 'alternate_swarming_dimensions' for 'Mac10.9 Tests' tester config
in waterfalls.py
3) manually run generate_buildbot_json.py and generated the multiple
swarming dimensions configs in chromium.mac.json

Bug: 805475
Change-Id: I4aeb4b67d085c58e777c59e9685e39e3d5504598
Reviewed-on: https://chromium-review.googlesource.com/898534
Commit-Queue: Shenghua Zhang <shenghuazhang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534879}