Add observer for BAAnnots from Javascript

fpdfsdk/cpdfsdk_baannot.cpp

Index: fpdfsdk/cpdfsdk_baannot.cpp
diff --git a/fpdfsdk/cpdfsdk_baannot.cpp b/fpdfsdk/cpdfsdk_baannot.cpp
index a2fd7c79bbae37d6f04b45f16ada3b6ab09a06cb..f5aa4e8fc72b4ecee419fb845c938733ef64ebcd 100644
--- a/fpdfsdk/cpdfsdk_baannot.cpp
+++ b/fpdfsdk/cpdfsdk_baannot.cpp
@@ -13,6 +13,22 @@
 #include "fpdfsdk/include/cpdfsdk_datetime.h"
 #include "fpdfsdk/include/fsdk_mgr.h"
 
+CPDFSDK_BAAnnot::Observer::Observer(CPDFSDK_BAAnnot** pWatchedPtr)
+    : m_pWatchedPtr(pWatchedPtr) {
+  (*m_pWatchedPtr)->AddObserver(this);
+}
+
+CPDFSDK_BAAnnot::Observer::~Observer() {
+  if (m_pWatchedPtr)
+    (*m_pWatchedPtr)->RemoveObserver(this);
+}
+
+void CPDFSDK_BAAnnot::Observer::OnAnnotDestroyed() {
+  ASSERT(m_pWatchedPtr);
+  *m_pWatchedPtr = nullptr;
+  m_pWatchedPtr = nullptr;
+}
+
 CPDFSDK_BAAnnot::CPDFSDK_BAAnnot(CPDF_Annot* pAnnot,
                                  CPDFSDK_PageView* pPageView)
     : CPDFSDK_Annot(pPageView), m_pAnnot(pAnnot) {}