Support clock drift compensation on Windows and add an API to allow enabling dynamically. BUG=issue773 TEST=unittest, trybots Review URL: https://webrtc-codereview.appspot.com/744007 git-svn-id: http://webrtc.googlecode.com/svn/trunk@2683 4adac7df-926f-26a2-2b94-8c16560cd09d