Add linux_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. Expand its testing.

BUG=353864

Review URL: https://codereview.chromium.org/246133002

git-svn-id: svn://svn.chromium.org/blink/trunk@172296 bbb929c8-8fbe-4397-9dbb-9b2b20218538