Add mac_gpu to the default set of try servers for Blink CLs.
Add mac_gpu to the default set of try servers for Blink CLs.

This configuration is successfully handling the experimental load from
the Chromium and Blink commit queues, and its hardware has just been
fully built out. Expand its testing.

BUG=353864

Review URL: https://codereview.chromium.org/275433009

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@173796 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs