Run blink unit tests under swarming on chromium.webkit mac bots.

R=jbudorick@chromium.org

Change-Id: I480364ec2aa241cdfc9b0f016cdb008f5226ecac
Reviewed-on: https://chromium-review.googlesource.com/821558


Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524409}