Lazy generate an "AP" when an Annot's hidden state changes

core/fpdfdoc/include/cpdf_annot.h

Index: core/fpdfdoc/include/cpdf_annot.h
diff --git a/core/fpdfdoc/include/cpdf_annot.h b/core/fpdfdoc/include/cpdf_annot.h
index 499934915568437dcd62524f387d79929d98ac59..4029bba4cc92d83acc23ce8eb2b95c7cfcadf350 100644
--- a/core/fpdfdoc/include/cpdf_annot.h
+++ b/core/fpdfdoc/include/cpdf_annot.h
@@ -63,6 +63,8 @@ class CPDF_Annot {
   CPDF_Form* GetAPForm(const CPDF_Page* pPage, AppearanceMode mode);
 
  private:
+  void GenerateAPIfNeeded();
+
   CPDF_Dictionary* const m_pAnnotDict;
   CPDF_Document* const m_pDocument;
   const CFX_ByteString m_sSubtype;