<chromium> Removed GUARDED_BY annotation from clock.h in webrtc.

This patch has been cherry-picked from upstream chromium,
as the GUARDED_BY annotation broke Qt builds on OSX with
older Xcode / clang versions.
Review URL: https://webrtc-codereview.appspot.com/14769004



Change-Id: Ib77d997f0c628e3bb378a06c427fbe938cd7dbc0
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>