Fix duplicate entries in sheriff-o-matic.

The key we were using for CTFailureGroup was not unique, so on an
update we'd overwrite one failure group with a completely different
failure group's data. We'd end up with having the data for one
group listed twice and the data for the other group disappearing.

Instead of using the sort_key from the server, give each group data
a unique key and us that as the group's key.

BUG=414559
NOTRY=true

Review URL: https://codereview.chromium.org/627233002

git-svn-id: svn://svn.chromium.org/blink/trunk@183289 bbb929c8-8fbe-4397-9dbb-9b2b20218538