[Sheriff-o-matic] Use likely_revisions instead of first_failing/last_passing

This will help in the transition to git hashes.

BUG=400408
NOTRY=true

Review URL: https://codereview.chromium.org/498523002

git-svn-id: svn://svn.chromium.org/blink/trunk@180933 bbb929c8-8fbe-4397-9dbb-9b2b20218538