Bad cast to CPDF_Document in FPDFEditEmbeddertest

The test, as written, is only correct for the non-xfa case, where
the FPDF_Document opaque type is equivalent to the CPDF_Document.
Under XFA, however, there is another layer of indirection that
the test harness needs to respect.

Sorry, I missed this the first time around when I signed off
on the CL that introduced this test.

Change-Id: Ida3579edf639fbfca2939dc151c0420247e90ab3
Reviewed-on: https://pdfium-review.googlesource.com/5657
Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/fpdfedit_embeddertest.cpp b/fpdfsdk/fpdfedit_embeddertest.cpp
index 4559ee3..b10dacf 100644
--- a/fpdfsdk/fpdfedit_embeddertest.cpp
+++ b/fpdfsdk/fpdfedit_embeddertest.cpp
@@ -26,7 +26,7 @@
  protected:
   FPDF_DOCUMENT CreateNewDocument() {
     document_ = FPDF_CreateNewDocument();
-    cpdf_doc_ = static_cast<CPDF_Document*>(document_);
+    cpdf_doc_ = CPDFDocumentFromFPDFDocument(document_);
     return document_;
   }