Fix ::~LogMessage to print as a string.

R=andrew@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/26949004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7525 4adac7df-926f-26a2-2b94-8c16560cd09d