Add APIs for limited use of document tagged code.

public/fpdf_structtree.h

Index: public/fpdf_structtree.h
diff --git a/public/fpdf_structtree.h b/public/fpdf_structtree.h
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+// Copyright 2016 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
+
+#ifndef PUBLIC_FPDF_STRUCTTREE_H_
+#define PUBLIC_FPDF_STRUCTTREE_H_
+
+// NOLINTNEXTLINE(build/include)
+#include "fpdfview.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Function: FPDF_StructTree_GetForPage
+//          Get the structure tree for a page.
+// Parameters:
+//          page        -   Handle to the page. Returned by FPDF_LoadPage
+//          function.
+// Return value:
+//          A handle to the structure tree or NULL on error.
+DLLEXPORT FPDF_STRUCTTREE STDCALL FPDF_StructTree_GetForPage(FPDF_PAGE page);
+
+// Function: FPDF_StructTree_Close
+//          Release the resource allocate by FPDF_StructTree_GetForPage.
+// Parameters:
+//          struct_tree -   Handle to the struct tree. Returned by
+//          FPDF_StructTree_LoadPage function.
+// Return value:
+//          NULL
+DLLEXPORT void STDCALL FPDF_StructTree_Close(FPDF_STRUCTTREE struct_tree);
+
+// Function: FPDF_StructTree_CountChildren
+//          Count the number of children for the structure tree.
+// Parameters:
+//          struct_tree -   Handle to the struct tree. Returned by
+//          FPDF_StructTree_LoadPage function.
+// Return value:
+//          The number of children, or -1 on error.
+DLLEXPORT int STDCALL
+FPDF_StructTree_CountChildren(FPDF_STRUCTTREE struct_tree);
+
+// Function: FPDF_StructTree_GetChildAtIndex
+//          Get a child in the structure tree.
+// Parameters:
+//          struct_tree -   Handle to the struct tree. Returned by
+//          FPDF_StructTree_LoadPage function.
+//          index       -   The index for the child, 0-based.
+// Return value:
+//          The child at the n-th index or NULL on error.
+DLLEXPORT FPDF_STRUCTELEMENT STDCALL
+FPDF_StructTree_GetChildAtIndex(FPDF_STRUCTTREE struct_tree, int index);
+
+// Function: FPDF_StructElement_GetAltText
+//          Get the alt text for a given element.
+// Parameters:
+//          struct_element -   Handle to the struct element.
+//          buffer         -   A buffer for output the alt text. May be NULL.
+//          buflen         -   The length of the buffer, in bytes. May be 0.
+// Return value:
+//          The number of bytes in the title, including the terminating NUL
+//          character. The number of bytes is returned regardless of the
+//          |buffer| and |buflen| parameters.
+// Comments:
+//          Regardless of the platform, the |buffer| is always in UTF-16LE
+//          encoding. The string is terminated by a UTF16 NUL character. If
+//          |buflen| is less than the required length, or |buffer| is NULL,
+//          |buffer| will not be modified.
+DLLEXPORT unsigned long STDCALL
+FPDF_StructElement_GetAltText(FPDF_STRUCTELEMENT struct_element,
+                              void* buffer,
+                              unsigned long buflen);
+
+// Function: FPDF_StructElement_CountChildren
+//          Count the number of children for the structure element.
+// Parameters:
+//          struct_element -   Handle to the struct element.
+// Return value:
+//          The number of children, or -1 on error.
+DLLEXPORT int STDCALL
+FPDF_StructElement_CountChildren(FPDF_STRUCTELEMENT struct_element);
+
+// Function: FPDF_StructElement_GetChildAtIndex
+//          Get a child in the structure element.
+// Parameters:
+//          struct_tree -   Handle to the struct element.
+//          index       -   The index for the child, 0-based.
+// Return value:
+//          The child at the n-th index or NULL on error.
+// Comments:
+//          If the child exists but is not an element, then this function will
+//          return NULL. This will also return NULL for out of bounds indices.
+DLLEXPORT FPDF_STRUCTELEMENT STDCALL
+FPDF_StructElement_GetChildAtIndex(FPDF_STRUCTELEMENT struct_element,
+                                   int index);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // PUBLIC_FPDF_STRUCTTREE_H_