Add failed_once category to Sheriff-O-Matic.

Failures that have only happened once, and only on one bot should be classified
differently than other failures-- it's quite possible that they're flakes.

NOTRY=true
BUG=405241

Review URL: https://codereview.chromium.org/546433003

git-svn-id: svn://svn.chromium.org/blink/trunk@181478 bbb929c8-8fbe-4397-9dbb-9b2b20218538