Simplify and guard access to WindowsRealTimeClock.

Addresses data race in get_time() causing incorrect timer roll-over
detection. This roll-over caused NTP timers to jump by 2^32
milliseconds affecting A/V sync and end-to-end delay calculations.

BUG=4206
R=dvyukov@google.com, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/33959004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@8112 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed