Enable incognito_session_only preferences

chrome/browser/extensions/extension_prefs_scope.h

 // Copyright (c) 2011 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTENSION_PREFS_SCOPE_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_PREFS_SCOPE_H_
 #pragma once
 
 // TODO(battre): get rid of this namespace, see
 // http://codereview.chromium.org/7065033/diff/3003/chrome/browser/extensions/extension_prefs_scope.h
 namespace extension_prefs_scope {
 
 // Scope for a preference.
 enum Scope {
   // Regular profile.
   kRegular,
   // Incognito profile; preference is persisted to disk and remains active
   // after a browser restart.
   kIncognitoPersistent,
-  // TODO(battre): add kIncognitoSession
+  // Incognito profile; preference is kept in memory and deleted when the
+  // incognito session is terminated.
+  kIncognitoSessionOnly
 };
 
 }  // extension_prefs_scope
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_PREFS_SCOPE_H_