Use override in more classes in fpdfsdk/

fpdfsdk/include/formfiller/FFL_CheckBox.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 FPDFSDK_INCLUDE_FORMFILLER_FFL_CHECKBOX_H_
 #define FPDFSDK_INCLUDE_FORMFILLER_FFL_CHECKBOX_H_
 
 #include "FFL_FormFiller.h"
 
 class CFFL_CheckBox : public CFFL_Button {
  public:
   CFFL_CheckBox(CPDFDoc_Environment* pApp, CPDFSDK_Widget* pWidget);
-  virtual ~CFFL_CheckBox();
+  ~CFFL_CheckBox() override;
 
-  virtual CPWL_Wnd* NewPDFWindow(const PWL_CREATEPARAM& cp,
-                                 CPDFSDK_PageView* pPageView);
-
-  virtual FX_BOOL OnKeyDown(CPDFSDK_Annot* pAnnot,
-                            FX_UINT nKeyCode,
-                            FX_UINT nFlags);
-  virtual FX_BOOL OnChar(CPDFSDK_Annot* pAnnot, FX_UINT nChar, FX_UINT nFlags);
-  virtual FX_BOOL OnLButtonUp(CPDFSDK_PageView* pPageView,
-                              CPDFSDK_Annot* pAnnot,
-                              FX_UINT nFlags,
-                              const CPDF_Point& point);
-
-  virtual FX_BOOL IsDataChanged(CPDFSDK_PageView* pPageView);
-  virtual void SaveData(CPDFSDK_PageView* pPageView);
+  // CFFL_Button
+  CPWL_Wnd* NewPDFWindow(const PWL_CREATEPARAM& cp,
+                         CPDFSDK_PageView* pPageView) override;
+  FX_BOOL OnKeyDown(CPDFSDK_Annot* pAnnot,
+                    FX_UINT nKeyCode,
+                    FX_UINT nFlags) override;
+  FX_BOOL OnChar(CPDFSDK_Annot* pAnnot, FX_UINT nChar, FX_UINT nFlags) override;
+  FX_BOOL OnLButtonUp(CPDFSDK_PageView* pPageView,
+                      CPDFSDK_Annot* pAnnot,
+                      FX_UINT nFlags,
+                      const CPDF_Point& point) override;
+  FX_BOOL IsDataChanged(CPDFSDK_PageView* pPageView) override;
+  void SaveData(CPDFSDK_PageView* pPageView) override;
 };
 
 #endif  // FPDFSDK_INCLUDE_FORMFILLER_FFL_CHECKBOX_H_