build_stages: Add logging to CancelObsoleteSlaveBuilds.

CancelObsoleteSlaveBuilds isn't always working in production, so add
logging to help with debugging.

BUG=chromium:855801
TEST=run_tests

Change-Id: I7d7a4db8d49f79610b46fcc8b62be64d4f543dc3
Reviewed-on: https://chromium-review.googlesource.com/1112855
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Mike Nichols <mikenichols@chromium.org>
1 file changed