Add code for new invalidation local ack system

sync/notifier/ack_handler.h

Index: sync/notifier/ack_handler.h
diff --git a/sync/notifier/ack_handler.h b/sync/notifier/ack_handler.h
new file mode 100644
index 0000000000000000000000000000000000000000..f1fc16fa702d294e5a96bb3188de3af85f3dc809
--- /dev/null
+++ b/sync/notifier/ack_handler.h
@@ -0,0 +1,42 @@
+// Copyright 2013 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_ACK_HANDLER_H_
+#define SYNC_NOTIFIER_ACK_HANDLER_H_
+
+#include <vector>
+
+#include "sync/base/sync_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 SYNC_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  // SYNC_NOTIFIER_ACK_HANDLER_H_