Add a SigninAllowed policy.

chrome/browser/app_controller_mac.mm

 // 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.
 
 #import "chrome/browser/app_controller_mac.h"
 
 #include "base/auto_reset.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/file_path.h"
@@ skipping 764 matching lines @@
           // http://code.google.com/p/chromium/issues/detail?id=43048 .
           //
           // TODO(akalin,viettrungluu): Figure out whether this method
           // can be prevented from being called if lastProfile is
           // NULL.
           if (!lastProfile) {
             LOG(WARNING)
                 << "NULL lastProfile detected -- not doing anything";
             break;
           }
-          enable = ![self keyWindowIsModal];
+          enable = lastProfile->IsSigninAllowed() &&
+              ![self keyWindowIsModal];
           [BrowserWindowController updateSigninItem:item
                                          shouldShow:enable
                                      currentProfile:lastProfile];
           break;
         }
         case IDC_FEEDBACK:
           enable = NO;
           break;
         default:
           enable = menuState_->IsCommandEnabled(tag) ?
@@ skipping 555 matching lines @@
 
 //---------------------------------------------------------------------------
 
 namespace app_controller_mac {
 
 bool IsOpeningNewWindow() {
   return g_is_opening_new_window;
 }
 
 }  // namespace app_controller_mac