Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/signin/signin_manager.h

Index: chrome/browser/signin/signin_manager.h
diff --git a/chrome/browser/signin/signin_manager.h b/chrome/browser/signin/signin_manager.h
index ce616eb4646a37b819dda29a93f49de825262e3d..d63df0cfd8733fb45631d97aa183097d5e341c17 100644
--- a/chrome/browser/signin/signin_manager.h
+++ b/chrome/browser/signin/signin_manager.h
@@ -24,6 +24,7 @@
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/prefs/public/pref_change_registrar.h"
+#include "base/prefs/public/pref_observer.h"
 #include "chrome/browser/profiles/profile_keyed_service.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
@@ -49,6 +50,7 @@ struct GoogleServiceSigninSuccessDetails {
 
 class SigninManager : public GaiaAuthConsumer,
                       public content::NotificationObserver,
+                      public PrefObserver,
                       public ProfileKeyedService {
  public:
   // Returns true if the cookie policy for the given profile allows cookies
@@ -146,6 +148,10 @@ class SigninManager : public GaiaAuthConsumer,
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
+  // PrefObserver
+  virtual void OnPreferenceChanged(PrefServiceBase* service,
+                                   const std::string& pref_name) OVERRIDE;
+
  protected:
   // Weak pointer to parent profile (protected so FakeSigninManager can access
   // it).