Only run one slave on the Mac 10.8 compile host
Only run one slave on the Mac 10.8 compile host

BUG=skia:
R=mtklein@google.com

Review URL: https://codereview.chromium.org/1156563002

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