Make SkSmallAllocator obey the RAII invariants and move to heap structures when needed.

Make SkSmallAllocator obey the RAII invariants and move to heap structures when needed.

The biggest change is to the API which allowed code to bypass the
destruction invariants. This destruction bypass feature was needed in
only one use, and is totally encapsulated using createWithIniterT.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2488523003

Review-Url: https://codereview.chromium.org/2488523003