Factoring out base class for trigger_multiple_dimensions.py.

This will be used by perf team for intercepting trigger call
to trigger on specific bot ids.

Bug: 758630
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ib5152748169ccd50d01dccc1f61979bcb967a481
Reviewed-on: https://chromium-review.googlesource.com/896365
Reviewed-by: Emily Hanley <[email protected]>
Reviewed-by: Ashley Enstad <[email protected]>
Reviewed-by: Kenneth Russell <[email protected]>
Commit-Queue: Emily Hanley <[email protected]>
Cr-Commit-Position: refs/heads/master@{#535001}
10 files changed