[luci-milo.cfg] Change GN logo

So that it's no longer the Chrome Infra logo.
This is a 3-sided throwing star, in the style of Chromium.

Change-Id: I21d4ac52d38475742ad40cf3eece2de1517bf39d
Reviewed-on: https://gn-review.googlesource.com/2401
Reviewed-by: Petr Hosek <phosek@google.com>