Refactor ModelTypePayloadMap and ObjectIdPayloadMap to StateMaps.

sync/internal_api/public/base/invalidation_state.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_INTERNAL_API_PUBLIC_BASE_INVALIDATION_STATE_H_
+#define SYNC_INTERNAL_API_PUBLIC_BASE_INVALIDATION_STATE_H_
+
+#include <string>
+
+namespace syncer {
+
+// Opaque class that represents an ack handle.
+// TODO(dcheng): This is just a refactoring change, so the class is empty for
+// the moment. It will be filled once we start implementing 'reminders'.
+class AckHandle {
+};
+
+struct InvalidationState {
+  std::string payload;
+  AckHandle ack_handle;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_BASE_INVALIDATION_STATE_H_