Fix failing build counts in sheriff-o-matic.

My recent fix for the uses of sheriff-o-matic's failing build
counts exposed that we no longer actually expose this data
in the alerts json. We just need to compute it from the
other data in the alert.

TBR since sheriff-o-matic is totally busted at the moment.

TBR=leviw@chromium.org
BUG=417256

Review URL: https://codereview.chromium.org/603793002

git-svn-id: svn://svn.chromium.org/blink/trunk@182635 bbb929c8-8fbe-4397-9dbb-9b2b20218538