Switch usage of _DEBUG macro to NDEBUG.

webrtc/base/stringutils.cc

Index: webrtc/base/stringutils.cc
diff --git a/webrtc/base/stringutils.cc b/webrtc/base/stringutils.cc
index 868e475f2d6fac229372bcea2412c8438f0b5cff..9580253d1b2f56e6a7e4a166720301d34217ca6e 100644
--- a/webrtc/base/stringutils.cc
+++ b/webrtc/base/stringutils.cc
@@ -77,11 +77,11 @@ size_t asccpyn(wchar_t* buffer, size_t buflen,
   } else if (srclen >= buflen) {
     srclen = buflen - 1;
   }
-#if _DEBUG
+#if !defined(NDEBUG)
   // Double check that characters are not UTF-8
   for (size_t pos = 0; pos < srclen; ++pos)
     RTC_DCHECK_LT(static_cast<unsigned char>(source[pos]), 128);
-#endif  // _DEBUG
+#endif
   std::copy(source, source + srclen, buffer);
   buffer[srclen] = 0;
   return srclen;