Enable Slug GM without need for defines

There is enough Slug infrastructure in place that this can be
turned on without the need for -D flags. This allows Slugs
to be tested in the CQ.

Change-Id: I1ff5cecb5acc061ff6e59702fff5af8aba92523a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/532757
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed