Reland: Make the CPDFXFA_App non-global

This reverts commit b685e6471075bebd26711bc183de3128f54932e3.

This CL changes CPDFXFA_App from a global object to a child of the CPDFXFA_Document objects.

BUG=pdfium:623

Review-Url: https://codereview.chromium.org/2418653003
diff --git a/fpdfsdk/fpdfdoc_unittest.cpp b/fpdfsdk/fpdfdoc_unittest.cpp
index fc85404..45beacf 100644
--- a/fpdfsdk/fpdfdoc_unittest.cpp
+++ b/fpdfsdk/fpdfdoc_unittest.cpp
@@ -35,8 +35,7 @@
 class CPDF_TestXFADocument : public CPDFXFA_Document {
  public:
   CPDF_TestXFADocument()
-      : CPDFXFA_Document(pdfium::MakeUnique<CPDF_TestDocument>(),
-                         CPDFXFA_App::GetInstance()) {}
+      : CPDFXFA_Document(pdfium::MakeUnique<CPDF_TestDocument>()) {}
 
   void SetRoot(CPDF_Dictionary* root) {
     reinterpret_cast<CPDF_TestDocument*>(GetPDFDoc())->SetRoot(root);