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

sync/notifier/invalidation_state_tracker.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.
 //
 // An InvalidationVersionTracker is an interface that handles getting
 // and setting (persisting) max invalidation versions.
 
 #ifndef SYNC_NOTIFIER_INVALIDATION_STATE_TRACKER_H_
 #define SYNC_NOTIFIER_INVALIDATION_STATE_TRACKER_H_
 
@@ skipping 13 matching lines @@
   virtual InvalidationVersionMap GetAllMaxVersions() const = 0;
 
   // |max_version| should be strictly greater than any existing max
   // version for |model_type|.
   virtual void SetMaxVersion(syncable::ModelType model_type,
                              int64 max_version) = 0;
 
   // Used by InvalidationClient for persistence. |state| is opaque data we can
   // present back to the client (e.g. after a restart) for it to bootstrap
   // itself.
+  // |state| is plain old data (not valid UTF8, embedded nulls, etc).
   virtual void SetInvalidationState(const std::string& state) = 0;
   virtual std::string GetInvalidationState() const = 0;
 
  protected:
   virtual ~InvalidationStateTracker() {}
 };
 
 }  // namespace sync_notifier
 
 #endif  // SYNC_NOTIFIER_INVALIDATION_STATE_TRACKER_H_