Enable Null Syncable ID which is different than Root ID.

sync/test/engine/test_id_factory.h

 // Copyright (c) 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.
 //
 // A tool making it easier to create IDs for unit testing.
 
 #ifndef SYNC_TEST_ENGINE_TEST_ID_FACTORY_H_
 #define SYNC_TEST_ENGINE_TEST_ID_FACTORY_H_
 
 #include <string>
 
 #include "base/strings/string_number_conversions.h"
 #include "sync/syncable/syncable_id.h"
 
 namespace syncer {
 
 class TestIdFactory {
  public:
   TestIdFactory() : next_value_(1337000) {}
   ~TestIdFactory() {}
 
   // Get the root ID.
-  static syncable::Id root() {
-    return syncable::Id();
-  }
+  static syncable::Id root() { return syncable::Id::GetRoot(); }
 
   // Make an ID from a number.  If the number is zero, return the root ID.
   // If the number is positive, create a server ID based on the value.  If
   // the number is negative, create a local ID based on the value.  This
   // is deterministic, and [FromNumber(X) == FromNumber(Y)] iff [X == Y].
   static syncable::Id FromNumber(int64 value) {
     if (value == 0)
       return root();
     else if (value < 0)
       return syncable::Id::CreateFromClientString(base::Int64ToString(value));
@@ skipping 27 matching lines @@
   int next_value() {
     return next_value_++;
   }
   int next_value_;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_TEST_ENGINE_TEST_ID_FACTORY_H_