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

webrtc/system_wrappers/source/logging.cc

@@ -55,7 +55,7 @@ bool LogMessage::Loggable(LoggingSeverity sev) {
 
 LogMessage::~LogMessage() {
   const std::string& str = print_stream_.str();
-  Trace::Add(WebRtcSeverity(severity_), kTraceUndefined, 0, str.c_str());
+  Trace::Add(WebRtcSeverity(severity_), kTraceUndefined, 0, "%s", str.c_str());
 }
 
 }  // namespace webrtc