Clean up some gm factory use.

Mostly use unique_ptr more consistently.

Change-Id: I6e11b272a7904eb662dea59b03fbc309a4cfc25d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233984
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
7 files changed