gm: use DEF_GM macro consistantly

Change-Id: I705180db106ec4515aaec5f347199b98b21976fa
Reviewed-on: https://skia-review.googlesource.com/c/186081
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
38 files changed