[graphite] Replace PrecompileChildPtr with a sk_sp<PrecompileBase>

I believe PrecompileChildPtr was originally added to parallel SkRuntimeEffect::ChildPtr. However, the Precompilation system has the shared PrecompileBase object while (in the main API) SkShader,
SkColorFilter and SkBlender only come together at SkFlattenable

Bug: b/332690425
Change-Id: Ied4d8c1d68521f48649369d14617f7fdd4283504
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/874246
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed