Enable parallel runs on ct.skia.org
Enable parallel runs on ct.skia.org

This is essentially a revert of https://codereview.chromium.org/1764683002/

BUG=skia:4506
TBR=benjaminwagner

Review URL: https://codereview.chromium.org/1834153002

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