Simplify and guard access to WindowsRealTimeClock.
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

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f938922c5c7021785769e9100ff3a5bc201196ff

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc