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/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;