Experimental APIs in public/fpdf_structtree.h changing

[Lei Zhang]

Hi everyone,

Just wanted to give a heads up that CL [1] is going to change a bunch
of experimental APIs in public/fpdf_structtree.h. e.g.

FPDF_OBJECT_TYPE type = FPDF_StructElement_Attr_GetType(attr, name);

becomes:

FPDF_STRUCTELEMENT_ATTR_VALUE value =
FPDF_StructElement_Attr_GetValue(attr, name);
FPDF_OBJECT_TYPE type = FPDF_StructElement_Attr_GetType(value);

[1] https://pdfium-review.googlesource.com/118691