Delete the extensions i18n message map when extension unload is received.

chrome/common/extensions/message_bundle.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/common/extensions/message_bundle.h"
 
 #include <string>
 #include <vector>
 
 #include "base/hash_tables.h"
@@ skipping 323 matching lines @@
 
 L10nMessagesMap* GetL10nMessagesMap(const std::string& extension_id) {
   ExtensionToL10nMessagesMap::iterator it =
       g_extension_to_messages_map.Get().messages_map.find(extension_id);
   if (it != g_extension_to_messages_map.Get().messages_map.end())
     return &(it->second);
 
   return NULL;
 }
 
+void EraseL10nMessagesMap(const std::string& extension_id) {
+  g_extension_to_messages_map.Get().messages_map.erase(extension_id);
+}
+
 }  // namespace extensions