Make macOS OS version numbers consistent

The "IsOS/IsAtLeastOS/IsAtMostOS" functions are driven from the Darwin
version number, which is always accurate. Adjust the
SysInfo::OperatingSystemVersionNumbers() function to return accurate
version numbers (or the best we can deduce) so that all version
numbers returned by helper functions are accurate and consistent.

Bug: 1101439
Bug: 1108832
Change-Id: I2ca92478d76bf572cc55875cba443f4978482d10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315490


Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791491}