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

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@8112 4adac7df-926f-26a2-2b94-8c16560cd09d

svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758