Add APIs for limited use of document tagged code.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 81b2c6173348193b5ba8f75fa5f9cebbc0db7d46..972469a2924af2a4e9fd0ecf92139b6acf8a7fca 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -97,6 +97,7 @@ static_library("pdfium") {
     "fpdfsdk/fpdf_flatten.cpp",
     "fpdfsdk/fpdf_progressive.cpp",
     "fpdfsdk/fpdf_searchex.cpp",
+    "fpdfsdk/fpdf_structtree.cpp",
     "fpdfsdk/fpdf_sysfontinfo.cpp",
     "fpdfsdk/fpdf_transformpage.cpp",
     "fpdfsdk/fpdfdoc.cpp",
@@ -124,6 +125,7 @@ static_library("pdfium") {
     "public/fpdf_progressive.h",
     "public/fpdf_save.h",
     "public/fpdf_searchex.h",
+    "public/fpdf_structtree.h",
     "public/fpdf_sysfontinfo.h",
     "public/fpdf_text.h",
     "public/fpdf_transformpage.h",
@@ -1795,6 +1797,7 @@ test("pdfium_embeddertests") {
     "core/fxge/ge/fx_ge_text_embeddertest.cpp",
     "fpdfsdk/fpdf_dataavail_embeddertest.cpp",
     "fpdfsdk/fpdf_flatten_embeddertest.cpp",
+    "fpdfsdk/fpdf_structtree_embeddertest.cpp",
     "fpdfsdk/fpdfdoc_embeddertest.cpp",
     "fpdfsdk/fpdfedit_embeddertest.cpp",
     "fpdfsdk/fpdfext_embeddertest.cpp",