Use std::set to track uniqueness in xfa_script_layoutpseudomodel.cpp
Use std::set to track uniqueness in xfa_script_layoutpseudomodel.cpp

Review-Url: https://codereview.chromium.org/2001743002

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium