Add observer for BAAnnots from Javascript

fpdfsdk/include/cpdfsdk_widget.h

Index: fpdfsdk/include/cpdfsdk_widget.h
diff --git a/fpdfsdk/include/cpdfsdk_widget.h b/fpdfsdk/include/cpdfsdk_widget.h
index 51116f9581421a9871f98a3be7b46d17259c2e88..531dc0aa148233ca055dc28bf987a4719d24ac0f 100644
--- a/fpdfsdk/include/cpdfsdk_widget.h
+++ b/fpdfsdk/include/cpdfsdk_widget.h
@@ -35,12 +35,12 @@ class CXFA_FFWidgetHandler;
 
 class CPDFSDK_Widget : public CPDFSDK_BAAnnot {
  public:
-  class Observer {
+  class Observer : public CPDFSDK_Annot::Observer {
    public:
     explicit Observer(CPDFSDK_Widget** pWatchedPtr);
-    ~Observer();
+    ~Observer() override;
 
-    void OnWidgetDestroyed();
+    void OnAnnotDestroyed() override;
 
    private:
     CPDFSDK_Widget** m_pWatchedPtr;
@@ -75,9 +75,6 @@ class CPDFSDK_Widget : public CPDFSDK_BAAnnot {
                  CPDFSDK_InterForm* pInterForm);
   ~CPDFSDK_Widget() override;
 
-  void AddObserver(Observer* observer);
-  void RemoveObserver(Observer* observer);
-
   bool IsSignatureWidget() const override;
   CPDF_Action GetAAction(CPDF_AAction::AActionType eAAT) override;
   FX_BOOL IsAppearanceValid() override;
@@ -180,7 +177,6 @@ class CPDFSDK_Widget : public CPDFSDK_BAAnnot {
   FX_BOOL m_bAppModified;
   int32_t m_nAppAge;
   int32_t m_nValueAge;
-  std::set<Observer*> m_Observers;
 
 #ifdef PDF_ENABLE_XFA
   mutable CXFA_FFWidget* m_hMixXFAWidget;