Add FX_OVERRIDE and use it for virtual functions of FX_FINAL classes.

core/include/fxcrt/fx_system.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
  
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef _FX_SYSTEM_H_
 #define _FX_SYSTEM_H_
 #define _FX_WIN32_DESKTOP_              1
 #define _FX_LINUX_DESKTOP_              4
@@ skipping 262 matching lines @@
 #define         FXSYS_Div(a, b) ((a) / (b))
 #define         FXSYS_MulDiv(a, b, c) ((a) * (b) / (c))
 #define         FXSYS_sqrt2(a, b) (FX_FLOAT)FXSYS_sqrt((a)*(a) + (b)*(b))
 #ifdef __cplusplus
 };
 
 #include "../../../third_party/numerics/safe_math.h"
 typedef base::CheckedNumeric<FX_DWORD> FX_SAFE_DWORD;
 typedef base::CheckedNumeric<FX_INT32> FX_SAFE_INT32;
 typedef base::CheckedNumeric<size_t>   FX_SAFE_SIZE_T;
+
 #if defined(__clang__) || _MSC_VER >= 1700
 #define FX_FINAL final
+#define FX_OVERRIDE override
 #elif defined(__GNUC__) && __cplusplus >= 201103 && \
       (__GNUC__ * 10000 + __GNUC_MINOR__ * 100) >= 40700
 #define FX_FINAL final
+#define FX_OVERRIDE override
 #else
 #define FX_FINAL
+#define FX_OVERRIDE
 #endif
 #endif
 #endif