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

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 57f9823aa9b0dacbe1ce85385f3e31a7ec228219..cdbdf9aeefc39c88e9a7f9c9216ac641a9213f4c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -229,6 +229,7 @@
     "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",
@@ -262,9 +263,11 @@
     "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" ]
 }