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

Committed: https://skia.googlesource.com/skia/+/d5dc657b8c3ac916f98005dafdedafe02f023449
Review-Url: https://codereview.chromium.org/2488523003