Change Timing::WallTimeNow to be static.

There's no need to construct a Timing object to call this method.
On Windows we were unnecessarily calling CreateWaitableTimer + CloseHandle but never actually using that waitable timer.

There's otherwise no change in functionality.

R=xians@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20869004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6624 4adac7df-926f-26a2-2b94-8c16560cd09d