Implement the TickTime::Now for mac using mach_absolute_time which is consistent even the user changes the system time. Review URL: https://webrtc-codereview.appspot.com/431004 git-svn-id: http://webrtc.googlecode.com/svn/trunk@1879 4adac7df-926f-26a2-2b94-8c16560cd09d