Teach garden-o-matic not to step on its own tail.

Because googlestorage may take non-trivially long time to fetch
results, garden-o-matic may occasionally initiate an update
cycle before the previous one completed, creating all kinds
of fun effects.

This is the simplest fix for now, just to stop an update cycle
when one is already in progress.

R=abarth@chromium.org, abarth, dpranke
BUG=307201

Review URL: https://codereview.chromium.org/149083002

git-svn-id: svn://svn.chromium.org/blink/trunk@165964 bbb929c8-8fbe-4397-9dbb-9b2b20218538