gm: virtual->override

Change-Id: I7782f1d08a03c725bed76078adfb9b84aad34a5f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228347
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
14 files changed