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

sync/notifier/sync_notifier_observer.h

 // 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.
 
 #ifndef SYNC_NOTIFIER_SYNC_NOTIFIER_OBSERVER_H_
 #define SYNC_NOTIFIER_SYNC_NOTIFIER_OBSERVER_H_
 #pragma once
 
 #include <string>
 
@@ skipping 10 matching lines @@
 
 class SyncNotifierObserver {
  public:
   SyncNotifierObserver() {}
   virtual ~SyncNotifierObserver() {}
 
   virtual void OnIncomingNotification(
       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
 
 #endif  // SYNC_NOTIFIER_SYNC_NOTIFIER_OBSERVER_H_