Use Mac 10.8 instead of 10.7 for CQ
Use Mac 10.8 instead of 10.7 for CQ

BUG=skia:2197
R=rmistry@google.com

Author: borenet@google.com

Review URL: https://codereview.chromium.org/174263002

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