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