gm: GMFactory returns a unique_ptr<>

Change-Id: If8a4d3cfdeaef58dd9924af430cfed834519b096
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/232496
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
9 files changed