Handle stale master alerts in SoM

We supported stale builders, but I have a patch in progress to
add stale master data (on a much shorter threshold) to the
builder alerts backend. Adding sheriff-o-matic support in advance
allows me to push that change without breaking anything.

Still need more test coverage. It may also be worth breaking
these out into their own type of failure instead of overloading
the builder failures, it just ends up being a lot more shaved
yak.

NOTRY=true

Review URL: https://codereview.chromium.org/618133003

git-svn-id: svn://svn.chromium.org/blink/trunk@183215 bbb929c8-8fbe-4397-9dbb-9b2b20218538