mac: Use Unified Logging when the deployment target is 10.12 or higher

Unified Logging (os_log) replaces ASL as the preferred system logging
interface in 10.12. It’s not available prior to 10.12, so ASL is still
used there. Attempting to use ASL with the deployment target set to
10.12 or higher produces deprecation warnings.

https://developer.apple.com/reference/os/1891852-logging

BUG=crashpad:120

Change-Id: Ibd65b181d495451aeb3ad7dd6a130a1f65544253
Reviewed-on: https://chromium-review.googlesource.com/366371
Reviewed-by: Robert Sesek <rsesek@chromium.org>