Enable Null Syncable ID which is different than Root ID.

sync/engine/get_commit_ids.cc

 // 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.
 
 #include "sync/engine/get_commit_ids.h"
 
 #include <set>
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 331 matching lines @@
   if (HaveItem(item_handle)) {
     // We've already added this item to the commit set, and so must have
     // already added the predecessors as well.
     return;
   }
   TryAddItem(ready_unsynced_set, item, result);
   if (item.GetIsDel())
     return;  // Deleted items have no predecessors.
 
   syncable::Id prev_id = item.GetPredecessorId();
-  while (!prev_id.IsRoot()) {
+  while (!prev_id.IsNull()) {
     syncable::Entry prev(trans_, syncable::GET_BY_ID, prev_id);
     CHECK(prev.good()) << "Bad id when walking predecessors.";
     if (!prev.GetIsUnsynced()) {
       // We're interested in "runs" of unsynced items.  This item breaks
       // the streak, so we stop traversing.
       return;
     }
     int64 handle = prev.GetMetahandle();
     if (HaveItem(handle)) {
       // We've already added this item to the commit set, and so must have
@@ skipping 194 matching lines @@
   // Add moves and creates, and prepend their uncommitted parents.
   traversal.AddCreatesAndMoves(ready_unsynced_set);
 
   // Add all deletes.
   traversal.AddDeletes(ready_unsynced_set);
 }
 
 }  // namespace
 
 }  // namespace syncer