SkOncePtr -> SkOnce

It's always nice to kill off a synchronization primitive.
And while less terse, I think this new code reads more clearly.

... and, SkOncePtr's tests were the only thing now using sk_num_cores()
outside of SkTaskGroup, so I've hidden it as static inside SkTaskGroup.cpp.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1953533002
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review-Url: https://codereview.chromium.org/1953533002