Implement new invalidations ack tracking system

chrome/browser/sync/glue/sync_backend_host_core.h

 // 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 CHROME_BROWSER_SYNC_GLUE_SYNC_BACKEND_HOST_CORE_H_
 #define CHROME_BROWSER_SYNC_GLUE_SYNC_BACKEND_HOST_CORE_H_
 
 #include "base/memory/ref_counted.h"
 
 #include "base/timer/timer.h"
@@ skipping 107 matching lines @@
   virtual void OnCryptographerStateChanged(
       syncer::Cryptographer* cryptographer) OVERRIDE;
   virtual void OnPassphraseTypeChanged(syncer::PassphraseType type,
                                        base::Time passphrase_time) OVERRIDE;
 
   // Forwards an invalidation state change to the sync manager.
   void DoOnInvalidatorStateChange(syncer::InvalidatorState state);
 
   // Forwards an invalidation to the sync manager.
   void DoOnIncomingInvalidation(
-      syncer::ObjectIdInvalidationMap invalidation_map);
+      const syncer::ObjectIdInvalidationMap& invalidation_map);
 
   // Note:
   //
   // The Do* methods are the various entry points from our
   // SyncBackendHost.  They are all called on the sync thread to
   // actually perform synchronous (and potentially blocking) syncapi
   // operations.
   //
   // Called to perform initialization of the syncapi on behalf of
   // SyncBackendHost::Initialize.
@@ skipping 147 matching lines @@
   const bool has_sync_setup_completed_;
 
   base::WeakPtrFactory<SyncBackendHostCore> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncBackendHostCore);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_GLUE_SYNC_BACKEND_HOST_CORE_H_