Move CPDFXFA_Context creation to FPDFDOC_InitFormFillEnvironment()

Currently the CPDFXFA_Context creation occurs in LoadDocumentImpl()
regardless of whether --disable-xfa has been passed to pdfium_test.
We want to defer the context creation to
FPDFDOC_InitFormFillEnvironment() and make it conditional to whether the
runtime flag has been passed.

Add a new field |xfa_disabled| to version 2 of _FPDF_FORMFILLINFO.
|xfa_disabled| was selected over |xfa_enabled| to allow for backwards
compatibility, as |xfa_disabled| defaults to false/0 upon
initialization.

This CL changes when the embedder gets a callback for when XFA is not
supported.

Fixed: pdfium:1433
Bug: pdfium:1433
Change-Id: I01da4fd75127815728d2c4dae357e4f22447a6c9
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/64770
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index d38227d..dc539dd 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -903,6 +903,8 @@
   FPDF_FORMFILLINFO_PDFiumTest form_callbacks = {};
 #ifdef PDF_ENABLE_XFA
   form_callbacks.version = 2;
+  form_callbacks.xfa_disabled =
+      options.disable_xfa || options.disable_javascript;
 #else   // PDF_ENABLE_XFA
   form_callbacks.version = 1;
 #endif  // PDF_ENABLE_XFA