Guard against double deletion of page views.

fpdfsdk/cpdfsdk_pageview.h

 // Copyright 2016 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
 
 #ifndef FPDFSDK_CPDFSDK_PAGEVIEW_H_
 #define FPDFSDK_CPDFSDK_PAGEVIEW_H_
 
 #include <memory>
@@ skipping 79 matching lines @@
     return m_fxAnnotArray;
   }
 
   int GetPageIndex() const;
   void LoadFXAnnots();
   void ClearFXAnnots();
   void SetValid(FX_BOOL bValid) { m_bValid = bValid; }
   FX_BOOL IsValid() { return m_bValid; }
   void SetLock(FX_BOOL bLocked) { m_bLocked = bLocked; }
   FX_BOOL IsLocked() { return m_bLocked; }
+
+  void SetBeingDestroyed() { m_bBeingDestroyed = true; }
+  bool IsBeingDestroyed() const { return m_bBeingDestroyed; }
+
 #ifndef PDF_ENABLE_XFA
   bool OwnsPage() const { return m_bOwnsPage; }
   void TakePageOwnership() { m_bOwnsPage = true; }
 #endif  // PDF_ENABLE_XFA
 
  private:
   int GetPageIndexForStaticPDF() const;
 
   CFX_Matrix m_curMatrix;
   UnderlyingPageType* const m_page;
   std::unique_ptr<CPDF_AnnotList> m_pAnnotList;
   std::vector<CPDFSDK_Annot*> m_fxAnnotArray;
   CPDFSDK_Document* const m_pSDKDoc;
   CPDFSDK_Annot::ObservedPtr m_pCaptureWidget;
 #ifndef PDF_ENABLE_XFA
   bool m_bOwnsPage;
 #endif  // PDF_ENABLE_XFA
   FX_BOOL m_bEnterWidget;
   FX_BOOL m_bExitWidget;
   FX_BOOL m_bOnWidget;
   FX_BOOL m_bValid;
   FX_BOOL m_bLocked;
+  bool m_bBeingDestroyed;
 };
 
 #endif  // FPDFSDK_CPDFSDK_PAGEVIEW_H_