Switch from _DEBUG to NDEBUG.

- NDEBUG is more widely used in Chromium
- _DEBUG is not defined on Mac in Chromium
- Leaving _DEBUG in for standalone debug builds for the benefit of
  third party libraries / Windows.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1721073004 .
diff --git a/core/include/fxcrt/fx_system.h b/core/include/fxcrt/fx_system.h
index f479868..41dc446 100644
--- a/core/include/fxcrt/fx_system.h
+++ b/core/include/fxcrt/fx_system.h
@@ -81,10 +81,6 @@
 // TODO(palmer): it should be a |size_t|, or at least unsigned.
 typedef int FX_STRSIZE;
 
-#if defined(DEBUG) && !defined(_DEBUG)
-#define _DEBUG
-#endif
-
 #ifndef TRUE
 #define TRUE 1
 #endif
@@ -104,7 +100,7 @@
 
 #define FXSYS_assert assert
 #ifndef ASSERT
-#ifdef _DEBUG
+#ifndef NDEBUG
 #define ASSERT FXSYS_assert
 #else
 #define ASSERT(a)
diff --git a/core/src/fxcrt/fx_basic_wstring.cpp b/core/src/fxcrt/fx_basic_wstring.cpp
index 136edd0..396dbc4 100644
--- a/core/src/fxcrt/fx_basic_wstring.cpp
+++ b/core/src/fxcrt/fx_basic_wstring.cpp
@@ -15,7 +15,7 @@
 
 namespace {
 
-#if defined(_DEBUG)
+#ifndef NDEBUG
 bool IsValidCodePage(FX_WORD codepage) {
   switch (codepage) {
     case 0:
diff --git a/fpdfsdk/src/javascript/global.cpp b/fpdfsdk/src/javascript/global.cpp
index 32995ff..e464339 100644
--- a/fpdfsdk/src/javascript/global.cpp
+++ b/fpdfsdk/src/javascript/global.cpp
@@ -57,7 +57,7 @@
   return (unsigned)FX_HashCode_String_GetW(main, FXSYS_wcslen(main));
 }
 
-#ifdef _DEBUG
+#ifndef NDEBUG
 class HashVerify {
  public:
   HashVerify();
diff --git a/xfa/src/fxfa/src/fm2js/xfa_fm2jscontext.cpp b/xfa/src/fxfa/src/fm2js/xfa_fm2jscontext.cpp
index 4b6fc0c..0ad73c1 100644
--- a/xfa/src/fxfa/src/fm2js/xfa_fm2jscontext.cpp
+++ b/xfa/src/fxfa/src/fm2js/xfa_fm2jscontext.cpp
@@ -3277,7 +3277,7 @@
         FXJSE_Value_Release(rgValues[i]);
       }
     } else if (FXJSE_Value_IsArray(argOne)) {
-#if defined(_DEBUG)
+#ifndef NDEBUG
       FXJSE_HVALUE lengthValue = FXJSE_Value_Create(hruntime);
       FXJSE_Value_GetObjectProp(argOne, "length", lengthValue);
       FXSYS_assert(FXJSE_Value_ToInteger(lengthValue) >= 3);
@@ -6458,7 +6458,7 @@
   if (argc == 1) {
     FXJSE_HVALUE argOne = args.GetValue(0);
     if (FXJSE_Value_IsArray(argOne)) {
-#if defined(_DEBUG)
+#ifndef NDEBUG
       FXJSE_HRUNTIME hruntime = pContext->GetScriptRuntime();
       FXJSE_HVALUE lengthValue = FXJSE_Value_Create(hruntime);
       FXJSE_Value_GetObjectProp(argOne, "length", lengthValue);
@@ -6484,7 +6484,7 @@
   if (argc == 1) {
     FXJSE_HVALUE argOne = args.GetValue(0);
     if (FXJSE_Value_IsArray(argOne)) {
-#if defined(_DEBUG)
+#ifndef NDEBUG
       FXJSE_HVALUE lengthValue = FXJSE_Value_Create(hruntime);
       FXJSE_Value_GetObjectProp(argOne, "length", lengthValue);
       FXSYS_assert(FXJSE_Value_ToInteger(lengthValue) >= 3);