Clean up CPDF_AnnotList.

core/src/fpdfdoc/doc_form.cpp

Index: core/src/fpdfdoc/doc_form.cpp
diff --git a/core/src/fpdfdoc/doc_form.cpp b/core/src/fpdfdoc/doc_form.cpp
index c155b947d942cda8bd92ae91671d642225d178ee..5cc2a66a89c3817a8b53dfed18708c3588cb5ada 100644
--- a/core/src/fpdfdoc/doc_form.cpp
+++ b/core/src/fpdfdoc/doc_form.cpp
@@ -818,7 +818,7 @@ CPDF_FormControl* CPDF_InterForm::GetControlAtPoint(CPDF_Page* pPage,
 }
 
 CPDF_FormControl* CPDF_InterForm::GetControlByDict(
-    CPDF_Dictionary* pWidgetDict) const {
+    const CPDF_Dictionary* pWidgetDict) const {
   const auto it = m_ControlMap.find(pWidgetDict);
   return it != m_ControlMap.end() ? it->second : nullptr;
 }