Run unit tests in a single thread when using DirectWrite
Run unit tests in a single thread when using DirectWrite

BUG=skia:2006
R=mtklein@google.com

Review URL: https://codereview.chromium.org/141483007

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