Enable Null Syncable ID which is different than Root ID.

sync/syncable/directory_backing_store.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/syncable/directory_backing_store.h"
 
 #include "build/build_config.h"
 
 #include <limits>
 
@@ skipping 1530 matching lines @@
        it != handles_map->end(); ++it) {
     EntryKernel* entry = it->second;
     bool is_duplicate_id = !(ids_set.insert(entry->ref(ID).value()).second);
     is_ok = is_ok && !is_duplicate_id;
   }
 
   IdsSet::iterator end = ids_set.end();
   for (Directory::MetahandlesMap::const_iterator it = handles_map->begin();
        it != handles_map->end(); ++it) {
     EntryKernel* entry = it->second;
-    bool parent_exists = (ids_set.find(entry->ref(PARENT_ID).value()) != end);
-    if (!parent_exists) {
-      return false;
+    if (!entry->ref(PARENT_ID).IsNull()) {
+      bool parent_exists = (ids_set.find(entry->ref(PARENT_ID).value()) != end);
+      if (!parent_exists) {
+        return false;
+      }
     }
   }
   return is_ok;
 }
 
 void DirectoryBackingStore::PrepareSaveEntryStatement(
     EntryTable table, sql::Statement* save_statement) {
   if (save_statement->is_valid())
     return;
 
@@ skipping 22 matching lines @@
   }
   query.append(" ) ");
   values.append(" )");
   query.append(values);
   save_statement->Assign(db_->GetUniqueStatement(
       base::StringPrintf(query.c_str(), "metas").c_str()));
 }
 
 }  // namespace syncable
 }  // namespace syncer