Add a SigninAllowed policy.

chrome/browser/ui/webui/ntp/new_tab_page_sync_handler.h

Index: chrome/browser/ui/webui/ntp/new_tab_page_sync_handler.h
diff --git a/chrome/browser/ui/webui/ntp/new_tab_page_sync_handler.h b/chrome/browser/ui/webui/ntp/new_tab_page_sync_handler.h
index 9ac909283d3cbce671210e3b71cd6af9b6520d8c..f906a0d35a49b4fd81993cf3fa96bc48c8e2e040 100644
--- a/chrome/browser/ui/webui/ntp/new_tab_page_sync_handler.h
+++ b/chrome/browser/ui/webui/ntp/new_tab_page_sync_handler.h
@@ -7,10 +7,13 @@
 
 #include <string>
 
+#include "chrome/browser/signin/signin_pref_observer.h"
 #include "chrome/browser/sync/profile_sync_service.h"
 #include "chrome/browser/sync/sync_ui_util.h"
 #include "content/public/browser/web_ui_message_handler.h"
 
+class SigninPrefObserver;
+
 namespace base {
 class ListValue;
 }
@@ -18,7 +21,8 @@ class ListValue;
 // Sends sync-state changes to the New Tab Page for UI updating and forwards
 // link clicks on the page to the sync service.
 class NewTabPageSyncHandler : public content::WebUIMessageHandler,
-                              public ProfileSyncServiceObserver {
+                              public ProfileSyncServiceObserver,
+                              public SigninPrefObserver {
  public:
   NewTabPageSyncHandler();
   virtual ~NewTabPageSyncHandler();
@@ -34,6 +38,9 @@ class NewTabPageSyncHandler : public content::WebUIMessageHandler,
   // ProfileSyncServiceObserver
   virtual void OnStateChanged() OVERRIDE;
 
+  // SigninPrefObserver
+  virtual void OnSigninAllowedPrefChange(bool signin_allowed) OVERRIDE;
+
  private:
   enum MessageType {
     HIDE,