Switch usage of _DEBUG macro to NDEBUG.

http://stackoverflow.com/a/29253284/5237416

BUG=None
R=tommi@webrtc.org
NOPRESUBMIT=true

Review URL: https://codereview.webrtc.org/1429513004

Cr-Commit-Position: refs/heads/master@{#10468}
diff --git a/webrtc/system_wrappers/include/logging.h b/webrtc/system_wrappers/include/logging.h
index 41c436b..d95c53e 100644
--- a/webrtc/system_wrappers/include/logging.h
+++ b/webrtc/system_wrappers/include/logging.h
@@ -131,7 +131,7 @@
     webrtc::LogMessage(__FILE__, __LINE__, sev).stream()
 
 // The _F version prefixes the message with the current function name.
-#if (defined(__GNUC__) && defined(_DEBUG)) || defined(WANT_PRETTY_LOG_F)
+#if (defined(__GNUC__) && !defined(NDEBUG)) || defined(WANT_PRETTY_LOG_F)
 #define LOG_F(sev) LOG(sev) << __PRETTY_FUNCTION__ << ": "
 #else
 #define LOG_F(sev) LOG(sev) << __FUNCTION__ << ": "