Fix -Wnon-virtual-dtor compiler warnings.

core/include/fxcrt/fx_basic.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 _FX_BASIC_H_
 #define _FX_BASIC_H_
 #ifndef _FX_SYSTEM_H_
 #include "fx_system.h"
@@ skipping 222 matching lines @@
     FX_DWORD                            m_LoadingPos;
 
     FX_LPCBYTE                          m_pLoadingBuf;
 
     FX_DWORD                            m_LoadingSize;
 };
 class IFX_BufferArchive
 {
 public:
     IFX_BufferArchive(FX_STRSIZE size);
-
+    virtual ~IFX_BufferArchive() { }
 
     virtual void                        Clear();
 
 
     FX_BOOL                                     Flush();
 
 
     FX_INT32                            AppendBlock(const void* pBuf, size_t size);
 
     FX_INT32                            AppendByte(FX_BYTE byte);
@@ skipping 10 matching lines @@
     FX_STRSIZE                          m_BufSize;
 
     FX_LPBYTE                           m_pBuffer;
 
     FX_STRSIZE                          m_Length;
 };
 class CFX_FileBufferArchive : public IFX_BufferArchive, public CFX_Object
 {
 public:
     CFX_FileBufferArchive(FX_STRSIZE size = 32768);
-    ~CFX_FileBufferArchive();
+    ~CFX_FileBufferArchive() override;
     virtual void                        Clear();
 
     FX_BOOL                                     AttachFile(IFX_StreamWrite *pFile, FX_BOOL bTakeover = FALSE);
 
     FX_BOOL                                     AttachFile(FX_LPCWSTR filename);
 
     FX_BOOL                                     AttachFile(FX_LPCSTR filename);
 private:
 
     virtual FX_BOOL                     DoWork(const void* pBuf, size_t size);
@@ skipping 1085 matching lines @@
     {
         return m_pObject == ref.m_pObject;
     }
 protected:
 
     CountedObj*                 m_pObject;
 };
 class IFX_Pause
 {
 public:
-
+    virtual ~IFX_Pause() { }
     virtual FX_BOOL     NeedToPauseNow() = 0;
 };
 class CFX_DataFilter : public CFX_Object
 {
 public:
 
     virtual ~CFX_DataFilter();
 
     void                        SetDestFilter(CFX_DataFilter* pFilter);
 
@@ skipping 243 matching lines @@
     FX_FLOAT c;
     FX_FLOAT d;
     FX_FLOAT e;
     FX_FLOAT f;
     FX_FLOAT g;
     FX_FLOAT h;
     FX_FLOAT i;
 };
 
 #endif