Fix -Wnon-virtual-dtor compiler warnings.

fpdfsdk/include/pdfwindow/PWL_Wnd.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 _PWL_WND_H_
 #define _PWL_WND_H_
 
 class IPWL_Provider;
@@ skipping 150 matching lines @@
 #define PWL_DEFAULT_WHITECOLOR                          CPWL_Color(COLORTYPE_GRAY,1)
 #define PWL_DEFAULT_HEAVYGRAYCOLOR                      CPWL_Color(COLORTYPE_GRAY,0.50)
 #define PWL_DEFAULT_LIGHTGRAYCOLOR                      CPWL_Color(COLORTYPE_GRAY,0.75)
 #define PWL_TRIANGLE_HALFLEN                            2.0f
 #define PWL_CBBUTTON_TRIANGLE_HALFLEN           3.0f
 #define PWL_INVALIDATE_INFLATE                          2
 
 class IPWL_SpellCheck
 {
 public:
+        virtual ~IPWL_SpellCheck() { }
         virtual FX_BOOL                                                 CheckWord(FX_LPCSTR sWord) = 0;
         virtual void                                                    SuggestWords(FX_LPCSTR sWord, CFX_ByteStringArray & sSuggest) = 0;      
 };
 
 class IPWL_Provider
 {
 public:
+        virtual ~IPWL_Provider() { }
+
         //get a matrix which map user space to CWnd client space
         virtual CPDF_Matrix                                             GetWindowMatrix(void* pAttachedData) = 0;
 
         /*
         0 L"&Undo\tCtrl+Z"
         1 L"&Redo\tCtrl+Shift+Z"
         2 L"Cu&t\tCtrl+X"
         3 L"&Copy\tCtrl+C"
         4 L"&Paste\tCtrl+V"
         5 L"&Delete"
         6  L"&Select All\tCtrl+A"
         */
         virtual CFX_WideString                                  LoadPopupMenuString(FX_INT32 nIndex) = 0;
 };
 
 class IPWL_FocusHandler
 {
 public:
+        virtual ~IPWL_FocusHandler() { }
         virtual void                                                    OnSetFocus(CPWL_Wnd* pWnd) = 0;
         virtual void                                                    OnKillFocus(CPWL_Wnd* pWnd) = 0;
 };
 
 struct PWL_CREATEPARAM
 {
 public:
         PWL_CREATEPARAM() : rcRectWnd(0,0,0,0),
                 pSystemHandler(NULL),
                 pFontMap(NULL),
@@ skipping 278 matching lines @@
 // #define VK_RETURN         0x0D
 // #define VK_ESCAPE         0x1B
 // #define VK_SPACE          0x20
 // #endif
 // 
 // #define VK_NONE                        0
 
 #endif // !defined(AFX_PWL_WND_H__D32812AD_A875_4E08_9D3C_0A57020987C6__INCLUDED_)