Store alerts in sheriff-o-matic's memcache.

TEST=Tools/GardeningServer/tests.py
BUG=401358
R=eseidel@chromium.org, ojan@chromium.org

Review URL: https://codereview.chromium.org/465233002

git-svn-id: svn://svn.chromium.org/blink/trunk@180153 bbb929c8-8fbe-4397-9dbb-9b2b20218538