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- 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/+/2285930Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791051}