Revert of Make the CPDFXFA_App non-global (patchset #3 id:80001 of https://codereview.chromium.org/2416753002/ )

Reason for revert:
https://build.chromium.org/p/client.pdfium/builders/linux_xfa_asan_lsan/builds/112/steps/embeddertests/logs/stdio

msan is sad.

Original issue's description:
> Make the CPDFXFA_App non-global
>
> This CL changes CPDFXFA_App from a global object to a child of the
> CPDFXFA_Document objects.
>
> BUG=pdfium:623
>
> Committed: https://pdfium.googlesource.com/pdfium/+/abefb79577b32d291d14d7e01a70f6f8cf213bd3

TBR=tsepez@chromium.org,thestig@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=pdfium:623

Review-Url: https://codereview.chromium.org/2412903004
diff --git a/fpdfsdk/fpdfdoc_unittest.cpp b/fpdfsdk/fpdfdoc_unittest.cpp
index 45beacf..fc85404 100644
--- a/fpdfsdk/fpdfdoc_unittest.cpp
+++ b/fpdfsdk/fpdfdoc_unittest.cpp
@@ -35,7 +35,8 @@
 class CPDF_TestXFADocument : public CPDFXFA_Document {
  public:
   CPDF_TestXFADocument()
-      : CPDFXFA_Document(pdfium::MakeUnique<CPDF_TestDocument>()) {}
+      : CPDFXFA_Document(pdfium::MakeUnique<CPDF_TestDocument>(),
+                         CPDFXFA_App::GetInstance()) {}
 
   void SetRoot(CPDF_Dictionary* root) {
     reinterpret_cast<CPDF_TestDocument*>(GetPDFDoc())->SetRoot(root);