mac: Make GetOSVersion() private to a cc file.

Everything calls GetOSVersionName() instead, so no need to expose that.
Since it uses deprecated functions, this will make it easier to rewrite
it in a future change.

BUG=webrtc:6027
NOTRY=true

Review-Url: https://codereview.webrtc.org/2117823002
Cr-Original-Commit-Position: refs/heads/master@{#13399}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0cf208aa28ed3c1b7a0425c96f2fdf8ac29eb25d
2 files changed