clock.h: Removed GUARDED_BY annotation as it breaks som builds.

BUG=N/A
TBR=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14769004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6547 4adac7df-926f-26a2-2b94-8c16560cd09d