Implement pref policy for disabling incognito mode.

chrome/browser/ui/browser.h

Index: chrome/browser/ui/browser.h
===================================================================
--- chrome/browser/ui/browser.h	(revision 72320)
+++ chrome/browser/ui/browser.h	(working copy)
@@ -1,4 +1,4 @@
-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -1110,6 +1110,9 @@
   // Keep track of when instant enabled changes.
   BooleanPrefMember instant_enabled_;
 
+  // Tracks the preference that controls whether incognito mode is allowed.
+  BooleanPrefMember incognito_mode_allowed_;
+
   // Indicates if command execution is blocked.
   bool block_command_execution_;