Make ipdf_formnotify a pure virtual interface

BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_common_config") {
   cflags = []
@@ skipping 294 matching lines @@
     "core/fpdfdoc/include/cpvt_line.h",
     "core/fpdfdoc/include/cpvt_secprops.h",
     "core/fpdfdoc/include/cpvt_section.h",
     "core/fpdfdoc/include/cpvt_word.h",
     "core/fpdfdoc/include/cpvt_wordplace.h",
     "core/fpdfdoc/include/cpvt_wordprops.h",
     "core/fpdfdoc/include/cpvt_wordrange.h",
     "core/fpdfdoc/include/fpdf_tagged.h",
     "core/fpdfdoc/include/ipdf_formnotify.h",
     "core/fpdfdoc/include/ipvt_fontmap.h",
-    "core/fpdfdoc/ipdf_formnotify.cpp",
     "core/fpdfdoc/tagged_int.h",
   ]
   configs += [ ":pdfium_core_config" ]
   deps = [
     ":fxcrt",
   ]
 }
 
 static_library("fpdfapi") {
   sources = [
@@ skipping 1422 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }