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This is a reland of 780e443e

This was blamed for compile failures that were actually due to
a roll of a broken macOS SDK.

Original change's description:
> Update core items for macOS Big Sur.
>
> - Update the base::mac::IsOS*() functions. Introduce a parallel set
>   of IsOS#() functions to match the IsOS10_#() functions. Update all
>   tests to match, and update callers where appropriate.
> - Turn on the CFAllocator OOM killer for Big Sur, as the
>   internals of CFAllocator have not changed.
>
> BUG=45650, 1101439
> TEST=base_unittests
>
> Change-Id: I57438eb025316e3cb899a0fa5aef35f4d297855f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285930
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Avi Drissman <avi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#791051}

Bug: 45650
Bug: 1101439
Tbr: mark@chromium.org
Change-Id: I3e5c0a18ad97a77811833534bc5f1b302cc6a73b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315162Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791131}