Revert of Remove the document tagged code as it is unused.

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 211 matching lines @@
     "core/fpdfdoc/ctypeset.h",
     "core/fpdfdoc/doc_action.cpp",
     "core/fpdfdoc/doc_basic.cpp",
     "core/fpdfdoc/doc_bookmark.cpp",
     "core/fpdfdoc/doc_form.cpp",
     "core/fpdfdoc/doc_formcontrol.cpp",
     "core/fpdfdoc/doc_formfield.cpp",
     "core/fpdfdoc/doc_link.cpp",
     "core/fpdfdoc/doc_metadata.cpp",
     "core/fpdfdoc/doc_ocg.cpp",
+    "core/fpdfdoc/doc_tagged.cpp",
     "core/fpdfdoc/doc_utils.cpp",
     "core/fpdfdoc/doc_utils.h",
     "core/fpdfdoc/doc_viewerPreferences.cpp",
     "core/fpdfdoc/doc_vt.cpp",
     "core/fpdfdoc/include/cpdf_aaction.h",
     "core/fpdfdoc/include/cpdf_action.h",
     "core/fpdfdoc/include/cpdf_actionfields.h",
     "core/fpdfdoc/include/cpdf_annot.h",
     "core/fpdfdoc/include/cpdf_annotlist.h",
     "core/fpdfdoc/include/cpdf_bookmark.h",
@@ skipping 13 matching lines @@
     "core/fpdfdoc/include/cpdf_occontext.h",
     "core/fpdfdoc/include/cpdf_variabletext.h",
     "core/fpdfdoc/include/cpdf_viewerpreferences.h",
     "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/ipvt_fontmap.h",
     "core/fpdfdoc/pdf_vt.h",
+    "core/fpdfdoc/tagged_int.h",
   ]
   configs += [ ":pdfium_core_config" ]
 }
 
 static_library("fpdfapi") {
   sources = [
     "core/fpdfapi/cpdf_modulemgr.cpp",
     "core/fpdfapi/cpdf_pagerendercontext.cpp",
     "core/fpdfapi/fpdf_cmaps/CNS1/Adobe-CNS1-UCS2_5.cpp",
     "core/fpdfapi/fpdf_cmaps/CNS1/B5pc-H_0.cpp",
@@ skipping 1353 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }