Adding R/W lock to SimulatedClock

This change makes the SimulatedClock class thread safe.

R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15569004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6251 4adac7df-926f-26a2-2b94-8c16560cd09d