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/unittest_main.cc b/webrtc/base/unittest_main.cc
index f952b2d..fb74e9e 100644
--- a/webrtc/base/unittest_main.cc
+++ b/webrtc/base/unittest_main.cc
@@ -78,12 +78,12 @@
     _CrtSetReportHook2(_CRT_RPTHOOK_INSTALL, TestCrtReportHandler);
   }
 
-#ifdef _DEBUG  // Turn on memory leak checking on Windows.
+#if !defined(NDEBUG)  // Turn on memory leak checking on Windows.
   _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF |_CRTDBG_LEAK_CHECK_DF);
   if (FLAG_crt_break_alloc >= 0) {
     _crtBreakAlloc = FLAG_crt_break_alloc;
   }
-#endif  // _DEBUG
+#endif
 #endif  // WEBRTC_WIN
 
   rtc::Filesystem::SetOrganizationName("google");