Enable Null Syncable ID which is different than Root ID.

sync/engine/syncer_util.cc

 // Copyright 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.
 
 #include "sync/engine/syncer_util.h"
 
 #include <algorithm>
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 103 matching lines @@
     if (local_entry.good()) {
       if (local_entry.GetId().ServerKnows()) {
         if (local_entry.GetId() != update_id) {
           // Case 2.
           LOG(WARNING) << "Duplicated client tag.";
           if (local_entry.GetId() < update_id) {
             // Signal an error; drop this update on the floor.  Note that
             // we don't server delete the item, because we don't allow it to
             // exist locally at all.  So the item will remain orphaned on
             // the server, and we won't pay attention to it.
-            return syncable::GetNullId();
+            return syncable::Id();
           }
         }
         // Target this change to the existing local entry; later,
         // we'll change the ID of the local entry to update_id
         // if needed.
         return local_entry.GetId();
       } else {
         // Case 3: We have a local entry with the same client tag.
         // We should change the ID of the local entry to the server entry.
         // This will result in an server ID with base version == 0, but that's
@@ skipping 510 matching lines @@
   if (update.version() < target->GetServerVersion()) {
     LOG(WARNING) << "Update older than current server version for "
                  << *target << " Update:"
                  << SyncerProtoUtil::SyncEntityDebugString(update);
     return VERIFY_SUCCESS;  // Expected in new sync protocol.
   }
   return VERIFY_UNDECIDED;
 }
 
 }  // namespace syncer