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This allows the generate_buildbot_json presubmit check to determine
whether or not it should validate that builders exist without having to
parse or scrape starlark source files.

Bug: 1056428
Change-Id: I9540512275d9c8235e92870670d60a2809b1fcdc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2268118
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782623}