sync: migrate invalidation state from syncable::Directory to InvalidationStorage

sync/notifier/sync_notifier_observer.h

Index: sync/notifier/sync_notifier_observer.h
diff --git a/sync/notifier/sync_notifier_observer.h b/sync/notifier/sync_notifier_observer.h
index 0d6cc021a7515949e1497addae82fabb806817c7..1130c9e3274903b0b413a2342a9c77925aa24867 100644
--- a/sync/notifier/sync_notifier_observer.h
+++ b/sync/notifier/sync_notifier_observer.h
@@ -28,10 +28,6 @@ class SyncNotifierObserver {
       const syncable::ModelTypePayloadMap& type_payloads,
       IncomingNotificationSource source) = 0;
   virtual void OnNotificationStateChange(bool notifications_enabled) = 0;
-
-  // TODO(nileshagrawal): Find a way to hide state handling inside the
-  // sync notifier implementation.
-  virtual void StoreState(const std::string& state) = 0;
 };
 
 }  // namespace sync_notifier