Fill in commit messages via gitiles API.
Fill in commit messages via gitiles API.

Screenshot:
  https://screenshot.googleplex.com/FhyczXuvrn.png

BUG=skia:
TBR=mtklein

Review URL: https://codereview.chromium.org/1067593004

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