Add Party Time to sheriff-o-matic

Been a good sheriff? Then you, too, can get some much-needed party time!
Adds party time when the list of failures for your tree is empty.

BUG=400403
NOTRY=true

Review URL: https://codereview.chromium.org/443233002

git-svn-id: svn://svn.chromium.org/blink/trunk@179672 bbb929c8-8fbe-4397-9dbb-9b2b20218538