gold: Add a black dot under the untriaged image.
gold: Add a black dot under the untriaged image.

Screenshot:
  https://screenshot.googleplex.com/5DyZ33Ud9bS.png

BUG=skia:

Review URL: https://codereview.chromium.org/1152963005

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