Use SkSmallAllocator for SamplerStage and Accessor. BUG=skia: GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4917 Change-Id: Id67d7c1cefa2aadfc706c56dd02d086120f99be3 Reviewed-on: https://skia-review.googlesource.com/4917 Commit-Queue: Herb Derby <herb@google.com> Reviewed-by: Mike Klein <mtklein@chromium.org>