Use override in more classes in fpdfsdk/

fpdfsdk/include/pdfwindow/PWL_Button.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_PDFWINDOW_PWL_BUTTON_H_
 #define FPDFSDK_INCLUDE_PDFWINDOW_PWL_BUTTON_H_
 
 #include "PWL_Wnd.h"
 
 class CPWL_Button : public CPWL_Wnd {
  public:
   CPWL_Button();
-  virtual ~CPWL_Button();
+  ~CPWL_Button() override;
 
-  virtual CFX_ByteString GetClassName() const;
-  virtual void OnCreate(PWL_CREATEPARAM& cp);
-  virtual FX_BOOL OnLButtonDown(const CPDF_Point& point, FX_DWORD nFlag);
-  virtual FX_BOOL OnLButtonUp(const CPDF_Point& point, FX_DWORD nFlag);
+  // CPWL_Wnd
+  CFX_ByteString GetClassName() const override;
+  void OnCreate(PWL_CREATEPARAM& cp) override;
+  FX_BOOL OnLButtonDown(const CPDF_Point& point, FX_DWORD nFlag) override;
+  FX_BOOL OnLButtonUp(const CPDF_Point& point, FX_DWORD nFlag) override;
 
  protected:
   FX_BOOL m_bMouseDown;
 };
 
 #endif  // FPDFSDK_INCLUDE_PDFWINDOW_PWL_BUTTON_H_