Split xfa_document_serialize into class files

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 126d2112019b4d399a731ffb4fa7bb85a4d0d0c5..67950952b7106812ffae844d3e2c1679c557cbc6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1362,6 +1362,10 @@ if (pdf_enable_xfa) {
       "xfa/fxfa/parser/cxfa_corner.h",
       "xfa/fxfa/parser/cxfa_data.cpp",
       "xfa/fxfa/parser/cxfa_data.h",
+      "xfa/fxfa/parser/cxfa_dataexporter.cpp",
+      "xfa/fxfa/parser/cxfa_dataexporter.h",
+      "xfa/fxfa/parser/cxfa_dataimporter.cpp",
+      "xfa/fxfa/parser/cxfa_dataimporter.h",
       "xfa/fxfa/parser/cxfa_document.cpp",
       "xfa/fxfa/parser/cxfa_document.h",
       "xfa/fxfa/parser/cxfa_document_parser.cpp",
@@ -1437,8 +1441,6 @@ if (pdf_enable_xfa) {
       "xfa/fxfa/parser/xfa_basic_data_packets.cpp",
       "xfa/fxfa/parser/xfa_document_datamerger_imp.cpp",
       "xfa/fxfa/parser/xfa_document_datamerger_imp.h",
-      "xfa/fxfa/parser/xfa_document_serialize.cpp",
-      "xfa/fxfa/parser/xfa_document_serialize.h",
       "xfa/fxfa/parser/xfa_layout_appadapter.cpp",
       "xfa/fxfa/parser/xfa_layout_appadapter.h",
       "xfa/fxfa/parser/xfa_layout_itemlayout.cpp",