Run probers on first startup and then periodically.
Run probers on first startup and then periodically.

As opposed to just running on timer.Tick, to avoid the delay in reporting probe metrics.

BUG=skia:

Review URL: https://codereview.chromium.org/1000723005

google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot