Enable Null Syncable ID which is different than Root ID.

sync/internal_api/base_node.cc

Index: sync/internal_api/base_node.cc
diff --git a/sync/internal_api/base_node.cc b/sync/internal_api/base_node.cc
index 57dbb22f72115e58dd8221f769afe2baf78bf5fc..b43c6ca79d0cf01ec165421aba328822aac553af 100644
--- a/sync/internal_api/base_node.cc
+++ b/sync/internal_api/base_node.cc
@@ -177,21 +177,21 @@ bool BaseNode::HasChildren() const {
 
 int64 BaseNode::GetPredecessorId() const {
   syncable::Id id_string = GetEntry()->GetPredecessorId();
-  if (id_string.IsRoot())
+  if (id_string.IsNull())
     return kInvalidId;
   return IdToMetahandle(GetTransaction()->GetWrappedTrans(), id_string);
 }
 
 int64 BaseNode::GetSuccessorId() const {
   syncable::Id id_string = GetEntry()->GetSuccessorId();
-  if (id_string.IsRoot())
+  if (id_string.IsNull())
     return kInvalidId;
   return IdToMetahandle(GetTransaction()->GetWrappedTrans(), id_string);
 }
 
 int64 BaseNode::GetFirstChildId() const {
   syncable::Id id_string = GetEntry()->GetFirstChildId();
-  if (id_string.IsRoot())
+  if (id_string.IsNull())
     return kInvalidId;
   return IdToMetahandle(GetTransaction()->GetWrappedTrans(), id_string);
 }