Make ipdf_formnotify a pure virtual interface

core/fpdfdoc/ipdf_formnotify.cpp

-// Copyright 2016 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
-
-#include "core/fpdfdoc/include/ipdf_formnotify.h"
-
-IPDF_FormNotify::~IPDF_FormNotify() {}
-
-int IPDF_FormNotify::BeforeValueChange(CPDF_FormField* pField,
-                                       const CFX_WideString& csValue) {
-  return 0;
-}
-
-void IPDF_FormNotify::AfterValueChange(CPDF_FormField* pField) {}
-
-int IPDF_FormNotify::BeforeSelectionChange(CPDF_FormField* pField,
-                                           const CFX_WideString& csValue) {
-  return 0;
-}
-
-void IPDF_FormNotify::AfterSelectionChange(CPDF_FormField* pField) {}
-
-void IPDF_FormNotify::AfterCheckedStatusChange(CPDF_FormField* pField) {}
-
-int IPDF_FormNotify::BeforeFormReset(CPDF_InterForm* pForm) {
-  return 0;
-}
-
-void IPDF_FormNotify::AfterFormReset(CPDF_InterForm* pForm) {}
-
-int IPDF_FormNotify::BeforeFormImportData(CPDF_InterForm* pForm) {
-  return 0;
-}
-
-void IPDF_FormNotify::AfterFormImportData(CPDF_InterForm* pForm) {}