Do not expose internal DS (iterator) of AreaCodeMaps

diff --git a/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.cc b/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.cc
index f8b013c..1e5a5ba 100644
--- a/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.cc
+++ b/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.cc

@@ -92,16 +92,12 @@
   }
   AreaCodeMaps::const_iterator it = available_maps_.find(filename);
   if (it == available_maps_.end()) {
-    it = LoadAreaCodeMapFromFile(filename);
-    if (it == available_maps_.end()) {
-      return NULL;
-    }
+    return LoadAreaCodeMapFromFile(filename);
   }
   return it->second;
 }
 
-PhoneNumberOfflineGeocoder::AreaCodeMaps::const_iterator
-PhoneNumberOfflineGeocoder::LoadAreaCodeMapFromFile(
+const AreaCodeMap* PhoneNumberOfflineGeocoder::LoadAreaCodeMapFromFile(
     const string& filename) const {
   const char** const prefix_language_code_pairs_end =
       prefix_language_code_pairs_ + prefix_language_code_pairs_size_;
@@ -114,9 +110,10 @@
     AreaCodeMap* const m = new AreaCodeMap();
     m->ReadAreaCodeMap(get_prefix_descriptions_(
             prefix_language_code_pair - prefix_language_code_pairs_));
-    return available_maps_.insert(AreaCodeMaps::value_type(filename, m)).first;
+    return available_maps_.insert(AreaCodeMaps::value_type(filename, m))
+        .first->second;
   }
-  return available_maps_.end();
+  return NULL;
 }
 
 string PhoneNumberOfflineGeocoder::GetCountryNameForNumber(

diff --git a/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.h b/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.h
index 9c09510..ee52090 100644
--- a/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.h
+++ b/cpp/src/phonenumbers/geocoding/phonenumber_offline_geocoder.h

@@ -116,7 +116,7 @@
             int prefix_language_code_pairs_size,
             prefix_descriptions_getter get_prefix_descriptions);
 
-  AreaCodeMaps::const_iterator LoadAreaCodeMapFromFile(
+  const AreaCodeMap* LoadAreaCodeMapFromFile(
       const string& filename) const ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);
 
   const AreaCodeMap* GetPhonePrefixDescriptions(