Use override in more classes in fpdfsdk/

fpdfsdk/include/fsdk_rendercontext.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_FSDK_RENDERCONTEXT_H_
 #define FPDFSDK_INCLUDE_FSDK_RENDERCONTEXT_H_
 
 #include "../../public/fpdf_progressive.h"
@@ skipping 13 matching lines @@
   CPDF_AnnotList* m_pAnnots;
   CPDF_RenderOptions* m_pOptions;
 #ifdef _WIN32_WCE
   CFX_DIBitmap* m_pBitmap;
   HBITMAP m_hBitmap;
 #endif
 };
 
 class IFSDK_PAUSE_Adapter : public IFX_Pause {
  public:
-  IFSDK_PAUSE_Adapter(IFSDK_PAUSE* IPause);
-  FX_BOOL NeedToPauseNow();
+  explicit IFSDK_PAUSE_Adapter(IFSDK_PAUSE* IPause);
+  ~IFSDK_PAUSE_Adapter() override;
+
+  FX_BOOL NeedToPauseNow() override;
 
  private:
   IFSDK_PAUSE* m_IPause;
 };
 
 #endif  // FPDFSDK_INCLUDE_FSDK_RENDERCONTEXT_H_