Add code for new invalidation local ack system

sync/notifier/ack_handler.cc

Index: sync/notifier/ack_handler.cc
diff --git a/sync/internal_api/public/base/cancelation_observer.cc b/sync/notifier/ack_handler.cc
similarity index 57%
copy from sync/internal_api/public/base/cancelation_observer.cc
copy to sync/notifier/ack_handler.cc
index f50b6a3c077722e6d02703fcef056e1d5e5cbace..3b31b2b7b2eec84a3da7b6a3eb3458f12d570a1c 100644
--- a/sync/internal_api/public/base/cancelation_observer.cc
+++ b/sync/notifier/ack_handler.cc
@@ -2,12 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "sync/internal_api/public/base/cancelation_observer.h"
+#include "sync/notifier/ack_handler.h"
+
+#include "sync/internal_api/public/base/invalidation.h"
 
 namespace syncer {
 
-CancelationObserver::CancelationObserver() {}
+AckHandler::AckHandler() {}
 
-CancelationObserver::~CancelationObserver() {}
+AckHandler::~AckHandler() {}
 
 }  // namespace syncer