Add a SigninAllowed policy.

chrome/browser/ui/cocoa/browser_window_controller.mm

Index: chrome/browser/ui/cocoa/browser_window_controller.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_controller.mm b/chrome/browser/ui/cocoa/browser_window_controller.mm
index 58c95ba2fe0d7cc7949248bc24955e1a2a023f3d..4f79d6f0c64b7a042c7f41663e7281e140261e93 100644
--- a/chrome/browser/ui/cocoa/browser_window_controller.mm
+++ b/chrome/browser/ui/cocoa/browser_window_controller.mm
@@ -1090,6 +1090,7 @@ enum {
         case IDC_SHOW_SIGNIN: {
           Profile* original_profile =
               browser_->profile()->GetOriginalProfile();
+          enable &= original_profile->IsSigninAllowed();

[sail, 2013/02/08 20:18:57]

Don't do this, if chrome::IsCommandEnabled() is not returning the correct value
then fix that function.

[Adrian Kuegel, 2013/02/11 16:47:30]

IsCommandEnabled should return the correct value, I am testing that with two
tests I added to the BrowserCommandController. So I will remove the code here.

           [BrowserWindowController updateSigninItem:item
                                          shouldShow:enable
                                      currentProfile:original_profile];