Add tree status display for sheriff-o-matic.

This essentially ports GardeningServer/scripts/treestatus.js to polymer.

NOTRY=true
R=ojan@chromium.org,abarth@chromium.org

Review URL: https://codereview.chromium.org/388333002

git-svn-id: svn://svn.chromium.org/blink/trunk@178298 bbb929c8-8fbe-4397-9dbb-9b2b20218538