Engage the presubmit warning for OS_MACOSX

OS_MACOSX will still work for code already in the tree, but warn
to prevent new instances from landing.

Bug: 1105907
Change-Id: Ia811e352ab7ec688bad5e8e7b31868777dd75c10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2328330
Commit-Queue: Avi Drissman <[email protected]>
Commit-Queue: Dirk Pranke <[email protected]>
Auto-Submit: Avi Drissman <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#793021}
2 files changed