Add a SigninAllowed policy.

chrome/browser/ui/cocoa/browser_window_controller.mm

 // Copyright 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.
 
 #import "chrome/browser/ui/cocoa/browser_window_controller.h"
 
 #include <cmath>
 #include <numeric>
 
 #include "base/command_line.h"
@@ skipping 1072 matching lines @@
             NSString* menuTitle = l10n_util::GetNSString(
                 [self inPresentationMode] ? IDS_EXIT_PRESENTATION_MAC :
                                             IDS_ENTER_PRESENTATION_MAC);
             [static_cast<NSMenuItem*>(item) setTitle:menuTitle];
           }
           break;
         }
         case IDC_SHOW_SIGNIN: {
           Profile* original_profile =
               browser_->profile()->GetOriginalProfile();
+          enable &= original_profile->IsSigninAllowed();
           [BrowserWindowController updateSigninItem:item
                                          shouldShow:enable
                                      currentProfile:original_profile];
           break;
         }
         default:
           // Special handling for the contents of the Text Encoding submenu. On
           // Mac OS, instead of enabling/disabling the top-level menu item, we
           // enable/disable the submenu's contents (per Apple's HIG).
           EncodingMenuController encoding_controller;
@@ skipping 1103 matching lines @@
 
 - (BOOL)supportsBookmarkBar {
   return [self supportsWindowFeature:Browser::FEATURE_BOOKMARKBAR];
 }
 
 - (BOOL)isTabbedWindow {
   return browser_->is_type_tabbed();
 }
 
 @end  // @implementation BrowserWindowController(WindowType)