Revert of SkOncePtr -> SkOnce (patchset #1 id:1 of https://codereview.chromium.org...

Revert of SkOncePtr -> SkOnce (patchset #1 id:1 of https://codereview.chromium.org/1949313003/ )

Reason for revert:
ok, I have no idea what's going on

Original issue's description:
> Reland of SkOncePtr -> SkOnce (patchset #1 id:1 of https://codereview.chromium.org/1945293004/ )
>
> Reason for revert:
> previous revert unneeded (I think)
>
> Original issue's description:
> > 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:
> >
> > Committed: https://skia.googlesource.com/skia/+/7eb33da7eede34c050b865dbb1b60c3dcea7191b
>
> 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:
>
> Committed: https://skia.googlesource.com/skia/+/82da9a8aa0ce648f36882830765b42e0ada6c0fa

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/1948313002