Rework garden-o-matic to fetch lists of builds from googlestorage properly.

TBR=abarth@chromium.org
BUG=261719

Review URL: https://codereview.chromium.org/23521006

git-svn-id: svn://svn.chromium.org/blink/trunk@157399 bbb929c8-8fbe-4397-9dbb-9b2b20218538