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}

webrtc/examples/objc/AppRTCDemo/ios/ARDAppDelegate.m

@@ -29,7 +29,7 @@
   ARDMainViewController *viewController = [[ARDMainViewController alloc] init];
   _window.rootViewController = viewController;
 
-#ifndef _DEBUG
+#if defined(NDEBUG)
   // In debug builds the default level is LS_INFO and in non-debug builds it is
   // disabled. Continue to log to console in non-debug builds, but only
   // warnings and errors.