Poll the masters every 30 mins.

It was 2 hrs before, which was arbitrary. If a bot blows up or
becomes otherwise unresponsive, this will decrease the turnaround
time for results on that bot to appear.

Since it "only" takes 40s - 60s to update the list and since it
ought to be non-blocking (different servlet), this should just
be an improvement.

BUG=260253

Review URL: https://chromiumcodereview.appspot.com/19267005

git-svn-id: svn://svn.chromium.org/blink/trunk@154256 bbb929c8-8fbe-4397-9dbb-9b2b20218538