split SkBBoxHierarchy into public/base types

This just does the boring work, keeping the
old type as an empty, now public, base type,
with all the existing methods on _Base.

Bug: skia:9796
Change-Id: I96d93d25955430c0586deca4bb562913d1d7815a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265447
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
8 files changed