Revert of Make the CPDFXFA_App non-global

fpdfsdk/fpdfformfill.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "public/fpdf_formfill.h"
 
 #include <memory>
 #include <vector>
@@ skipping 240 matching lines @@
   // environment into the XFADocument and, that could get weird.
   if (pDocument->GetFormFillEnv())
     return pDocument->GetFormFillEnv();
 #endif
 
   CPDFSDK_FormFillEnvironment* pFormFillEnv =
       new CPDFSDK_FormFillEnvironment(pDocument, formInfo);
 
 #ifdef PDF_ENABLE_XFA
   pDocument->SetFormFillEnv(pFormFillEnv);
+  CPDFXFA_App::GetInstance()->AddFormFillEnv(pFormFillEnv);
 #endif  // PDF_ENABLE_XFA
 
   return pFormFillEnv;
 }
 
 DLLEXPORT void STDCALL
 FPDFDOC_ExitFormFillEnvironment(FPDF_FORMHANDLE hHandle) {
   if (!hHandle)
     return;
 
   CPDFSDK_FormFillEnvironment* pFormFillEnv =
       HandleToCPDFSDKEnvironment(hHandle);
 
 #ifdef PDF_ENABLE_XFA
+  CPDFXFA_App::GetInstance()->RemoveFormFillEnv(pFormFillEnv);
+
   // Reset the focused annotations and remove the SDK document from the
   // XFA document.
   pFormFillEnv->ClearAllFocusedAnnots();
   // If the document was closed first, it's possible the XFA document
   // is now a nullptr.
   if (pFormFillEnv->GetXFADocument())
     pFormFillEnv->GetXFADocument()->SetFormFillEnv(nullptr);
 #endif  // PDF_ENABLE_XFA
 
   delete pFormFillEnv;
@@ skipping 456 matching lines @@
                                     pFormFillEnv);
     }
   } else {
     if (aa.ActionExist(CPDF_AAction::ClosePage)) {
       CPDF_Action action = aa.GetAction(CPDF_AAction::ClosePage);
       pActionHandler->DoAction_Page(action, CPDF_AAction::ClosePage,
                                     pFormFillEnv);
     }
   }
 }