Use override in more classes in fpdfsdk/

fpdfsdk/include/pdfwindow/PWL_Button.h

Index: fpdfsdk/include/pdfwindow/PWL_Button.h
diff --git a/fpdfsdk/include/pdfwindow/PWL_Button.h b/fpdfsdk/include/pdfwindow/PWL_Button.h
index 363556762fb780dd3d78be7d960a6e0af9becde2..28a026c3bf3e6105326c206a63254c194bc56eab 100644
--- a/fpdfsdk/include/pdfwindow/PWL_Button.h
+++ b/fpdfsdk/include/pdfwindow/PWL_Button.h
@@ -12,12 +12,13 @@
 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;