Use stale_builder_alerts in sheriff-o-matic

Start using the stale_builder_alerts from the alerts json
instead of building it up ourselve by iterating over all the
builders on the client. This greatly simplifies the work
we need to do during update, and paves the way to removing
the builder info from the alerts json and into its own
stream.

This also removes the special casing of the blink waterfall
so Chromium sheriffs will also see stalled bots.

NOTRY=true
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/598773002

git-svn-id: svn://svn.chromium.org/blink/trunk@182632 bbb929c8-8fbe-4397-9dbb-9b2b20218538