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

core/src/fpdfapi/fpdf_font/ttgsubtable.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 TTGSUBTable_H
 #define TTGSUBTable_H
 #include "../../fx_freetype.h"
 #include "../../../include/fxcrt/fx_basic.h"
@@ skipping 391 matching lines @@
     FX_BOOL     m_bFeautureMapLoad;
     bool loaded;
     struct tt_gsub_header header;
     struct TScriptList ScriptList;
     struct TFeatureList FeatureList;
     struct TLookupList LookupList;
 };
 class CFX_GSUBTable FX_FINAL : public IFX_GSUBTable, public CFX_Object
 {
 public:
-    virtual void        Release()
+    virtual void        Release() FX_OVERRIDE
     {
         delete this;
     }
-    virtual FX_BOOL GetVerticalGlyph(FX_DWORD glyphnum, FX_DWORD* vglyphnum);
+    virtual FX_BOOL GetVerticalGlyph(FX_DWORD glyphnum, FX_DWORD* vglyphnum) FX_OVERRIDE;
     CFX_CTTGSUBTable m_GsubImp;
 };
 #endif