Fix Windows crash_generation_server for debug builds without -D_DEBUG

Debug Gecko builds don't build with -D_DEBUG, so the ifdef in
crash_generation_server doesn't work right. The MSDN documentation for
assert says that it's enabled based on the absence of the NDEBUG define,
so using that seems sensible.

R=thestig@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1398453002 .

src/client/windows/crash_generation/crash_generation_server.cc

@@ -85,7 +85,7 @@ static bool IsClientRequestValid(const ProtocolMessage& msg) {
           msg.assert_info != NULL);
 }
 
-#ifdef _DEBUG
+#ifndef NDEBUG
 static bool CheckForIOIncomplete(bool success) {
   // We should never get an I/O incomplete since we should not execute this
   // unless the operation has finished and the overlapped event is signaled. If