Add APIs for limited use of document tagged code.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 84a415cd4d442be2c91217b8cb324ddb33bdacd9..29572e72c581b3173e6a7b4b084b7b1d8bdb6710 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",
@@ -1797,6 +1799,7 @@ 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",