Fix potential deadlock in webrtc/system_wrappers/source/logging_unittest.cc.

crit_ should not be held while calling Trace.

BUG=3003
R=henrike@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17859005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6616 4adac7df-926f-26a2-2b94-8c16560cd09d