Introduce a layering in the invalidation component as public and impl

components/invalidation/ack_handler.h

-// Copyright 2014 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 COMPONENTS_INVALIDATION_ACK_HANDLER_H_
-#define COMPONENTS_INVALIDATION_ACK_HANDLER_H_
-
-#include <vector>
-
-#include "components/invalidation/invalidation_export.h"
-
-namespace invalidation {
-class ObjectId;
-}  // namespace invalidation
-
-namespace syncer {
-
-class AckHandle;
-
-// An interface for classes that keep track of invalidation acknowledgements.
-//
-// We don't expect to support more than one "real" implementation of AckHandler,
-// but this interface is very useful for testing and implementation hiding.
-class INVALIDATION_EXPORT AckHandler {
- public:
-  AckHandler();
-  virtual ~AckHandler() = 0;
-
-  // Record the local acknowledgement of an invalidation identified by |handle|.
-  virtual void Acknowledge(const invalidation::ObjectId& id,
-                           const AckHandle& handle) = 0;
-
-  // Record the drop of an invalidation identified by |handle|.
-  virtual void Drop(const invalidation::ObjectId& id,
-                    const AckHandle& handle) = 0;
-};
-
-}  // namespace syncer
-
-#endif  // COMPONENTS_INVALIDATION_ACK_HANDLER_H_