Revert of Run unit tests in a single thread when using DirectWrite (https://codereview.chromium.org/141483007/)
Revert of Run unit tests in a single thread when using DirectWrite (https://codereview.chromium.org/141483007/)

Reason for revert:
Threading issue is supposed to be fixed.

BUG=skia:2006
Original issue's description:
> Run unit tests in a single thread when using DirectWrite
>
> BUG=skia:2006
> R=mtklein@google.com
>
> Committed: https://skia.googlesource.com/buildbot/+/2c8eff2

R=caryclark@google.com, robertphillips@google.com, bungeman@google.com, mtklein@google.com, bungeman@chromium.org

Author: borenet@google.com

Review URL: https://codereview.chromium.org/259043003

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