Caused by https://chromium-review.googlesource.com/1061597 . Fix in progress.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/78fd87097942a66b7e09aba20255e7e5779f21c7

commit 78fd87097942a66b7e09aba20255e7e5779f21c7
Author: Kenneth Russell <kbr@chromium.org>
Date: Thu May 17 01:15:52 2018

Fix generate_buildbot_json_coveragetest.

Actually cover the code which reads luci-milo.cfg.

Bug:  843842 
No-Try: True
Change-Id: Icd0fcaa2b6037c1ae3560a8ddb6e6ba777b3ac80
Tbr: thakis@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1063270
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559380}
[modify] https://crrev.com/78fd87097942a66b7e09aba20255e7e5779f21c7/testing/buildbot/generate_buildbot_json.py
[modify] https://crrev.com/78fd87097942a66b7e09aba20255e7e5779f21c7/testing/buildbot/generate_buildbot_json_unittest.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5a89036b61de53c4083da98659cfb63c53923784

commit 5a89036b61de53c4083da98659cfb63c53923784
Author: Kenneth Russell <kbr@chromium.org>
Date: Thu May 17 22:33:41 2018

Make generate_buildbot_json_coveragetest mandatory.

coverage is now part of Chromium's vpython spec. This script is
crucial to Chrome's infrastructure, and changes should not be allowed
unless the standard of code coverage is maintained.

Bug:  843842 ,  662541 ,  792130 
Change-Id: I9164fe331045d13e08c1e1d40c79a01bac368ff9
Reviewed-on: https://chromium-review.googlesource.com/1063257
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559711}
[modify] https://crrev.com/5a89036b61de53c4083da98659cfb63c53923784/testing/buildbot/PRESUBMIT.py