Enable Null Syncable ID which is different than Root ID.

sync/internal_api/write_node.cc

Index: sync/internal_api/write_node.cc
diff --git a/sync/internal_api/write_node.cc b/sync/internal_api/write_node.cc
index b1f08725ac20d9eb8e98998d0622c557b17298a6..4aab598bc319be12c40ce307647f8e14bb4a23eb 100644
--- a/sync/internal_api/write_node.cc
+++ b/sync/internal_api/write_node.cc
@@ -461,7 +461,7 @@ bool WriteNode::SetPosition(const BaseNode& new_parent,
   // Filter out redundant changes if both the parent and the predecessor match.
   if (new_parent_id == entry_->GetParentId()) {
     const syncable::Id& old = entry_->GetPredecessorId();
-    if ((!predecessor && old.IsRoot()) ||
+    if ((!predecessor && old.IsNull()) ||
         (predecessor && (old == predecessor->GetEntry()->GetId()))) {
       return true;
     }