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This CL updates the find_bad_builds.py script to look at the buildbucket
data to decide which builds to cancel.

CLs which land in chromium/src can often cause issues with the
Chromium CQ, for a variety of reasons. Most commonly, CLs can cause
test runtimes to dramatically increase, which overloads our pools.

One way to mitigate issues like these is to cancel builds which contain
the bad CLs, to alleviate the load on our machine pools. This CL adds a
fairly simple script which finds build which contain a specific CL.
There are many possible features and additions to this script, but the
simple version in this CL should be useful enough for now.

Bug: 1137072
Change-Id: Ib163a4bbb78c17c11ecb17c836a8e62da2b66453
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464165
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822887}