Fix CData parsing in CFDE_XMLSyntaxParser.

This CL splits the handling of CData sections out to an individual phase
of the parser. This fixes the issue with the CData parser getting confused
by < characters inside the data section.

BUG=pdfium:90

Review URL: https://codereview.chromium.org/1842633004
diff --git a/BUILD.gn b/BUILD.gn
index a21c0da..6780f2b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1613,6 +1613,7 @@
   include_dirs = []
   if (pdf_enable_xfa) {
     sources += [
+      "xfa/fde/xml/fde_xml_imp_unittest.cpp",
       "xfa/fxbarcode/pdf417/BC_PDF417HighLevelEncoder_unittest.cpp",
       "xfa/fxfa/parser/xfa_utils_imp_unittest.cpp",
     ]