Revert of SkOncePtr -> SkOnce (patchset #4 id:60001 of https://codereview.chromium.org/1953533002/ )

Reason for revert:
broken the Mac and Linux builders, e.g.:

https://build.chromium.org/p/chromium/builders/Mac/builds/15151
https://build.chromium.org/p/chromium/builders/Linux%20x64/builds/19052

Original issue's description:
> 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
>
> Committed: https://skia.googlesource.com/skia/+/9bd3fc7fa9bb7cad050bd619aa93d4c48ebb5c02

TBR=herb@google.com,mtklein@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review-Url: https://codereview.chromium.org/1945293004
20 files changed