Update core items for macOS 10.15.

- The expected Darwin/OS version correspondence is valid.
- Update the base::mac::IsOS*() functions. Remove the 10.9
  functions, as 10.10 is the current minimum, and remove
  support for pre-10.13 SDKs as the 10.13 SDK is required.
  Remove support for IsAtLeastOS10_*() for the minimum
  required OS version, as it is always true.
- Turn on the CFAllocator OOM killer for 10.15, as the
  internals of CFAllocator have not changed.

BUG=45650
TEST=base_unittests

Change-Id: Iee57ac666a5572349643e6965ccee11f8657c987
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1647435
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666801}