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/base/opensslstreamadapter.cc b/webrtc/base/opensslstreamadapter.cc
index 67ed5db..9c3a09e 100644
--- a/webrtc/base/opensslstreamadapter.cc
+++ b/webrtc/base/opensslstreamadapter.cc
@@ -994,7 +994,7 @@
     return NULL;
   }
 
-#ifdef _DEBUG
+#if !defined(NDEBUG)
   SSL_CTX_set_info_callback(ctx, OpenSSLAdapter::SSLInfoCallback);
 #endif