Fix -Wnon-virtual-dtor compiler warnings.

core/include/fxge/fpf.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_PALTFORM_DEVICE_H_
 #define _FX_PALTFORM_DEVICE_H_
 class IFPF_DeviceModule;
 class IFPF_FontMgr;
 class IFPF_Font;
 class IFPF_DeviceModule
 {
 public:
+    virtual ~IFPF_DeviceModule() { }
     virtual void                                Destroy() = 0;
     virtual IFPF_FontMgr*               GetFontMgr() = 0;
 };
 IFPF_DeviceModule*      FPF_GetDeviceModule();
 #define FPF_MATCHFONT_REPLACEANSI               1
 FX_DEFINEHANDLE(FPF_HFONT);
 class IFPF_Font
 {
 public:
     virtual void                        Release() = 0;
     virtual IFPF_Font*          Retain() = 0;
     virtual FPF_HFONT           GetHandle() = 0;
     virtual CFX_ByteString      GetFamilyName() = 0;
     virtual CFX_WideString      GetPsName() = 0;
     virtual FX_DWORD            GetFontStyle() const = 0;
     virtual FX_BYTE                     GetCharset() const = 0;
 
     virtual FX_INT32            GetGlyphIndex(FX_WCHAR wUnicode) = 0;
     virtual FX_INT32            GetGlyphWidth(FX_INT32 iGlyphIndex) = 0;
 
     virtual FX_INT32            GetAscent() const = 0;
     virtual FX_INT32            GetDescent() const = 0;
 
     virtual FX_BOOL                     GetGlyphBBox(FX_INT32 iGlyphIndex, FX_RECT &rtBBox) = 0;
     virtual FX_BOOL                     GetBBox(FX_RECT &rtBBox) = 0;
 
     virtual FX_INT32            GetHeight() const = 0;
     virtual FX_INT32            GetItalicAngle() const = 0;
     virtual FX_DWORD            GetFontData(FX_DWORD dwTable, FX_LPBYTE pBuffer, FX_DWORD dwSize) = 0;
+
+protected:
+    ~IFPF_Font() { }
 };
 class IFPF_FontMgr
 {
 public:
+    virtual ~IFPF_FontMgr() { }
     virtual void                        LoadSystemFonts() = 0;
     virtual void                        LoadPrivateFont(IFX_FileRead* pFontFile) = 0;
     virtual void                        LoadPrivateFont(FX_BSTR bsFileName) = 0;
     virtual void                        LoadPrivateFont(FX_LPVOID pBuffer, size_t szBuffer) = 0;
 
     virtual IFPF_Font*          CreateFont(FX_BSTR bsFamilyname, FX_BYTE charset, FX_DWORD dwStyle, FX_DWORD dwMatch = 0) = 0;
 };
 #endif