Add observer for BAAnnots from Javascript

fpdfsdk/include/cpdfsdk_baannot.h

Index: fpdfsdk/include/cpdfsdk_baannot.h
diff --git a/fpdfsdk/include/cpdfsdk_baannot.h b/fpdfsdk/include/cpdfsdk_baannot.h
index 4575bfe087c4e54264407b516752c92d9622275c..9d56bf424cced53c0d0d55fc7d96eb9f02620c80 100644
--- a/fpdfsdk/include/cpdfsdk_baannot.h
+++ b/fpdfsdk/include/cpdfsdk_baannot.h
@@ -24,6 +24,17 @@ class CPDFSDK_PageView;
 
 class CPDFSDK_BAAnnot : public CPDFSDK_Annot {
  public:
+  class Observer : public CPDFSDK_Annot::Observer {
+   public:
+    explicit Observer(CPDFSDK_BAAnnot** pWatchedPtr);
+    ~Observer() override;
+
+    void OnAnnotDestroyed() override;
+
+   private:
+    CPDFSDK_BAAnnot** m_pWatchedPtr;
+  };
+
   CPDFSDK_BAAnnot(CPDF_Annot* pAnnot, CPDFSDK_PageView* pPageView);
   ~CPDFSDK_BAAnnot() override;