Enable Null Syncable ID which is different than Root ID.

sync/syncable/syncable_id.cc

 // 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.
 
 #include "sync/syncable/syncable_id.h"
 
 #include <iosfwd>
 
+#include "base/logging.h"
 #include "base/values.h"
 
 using std::ostream;
 using std::string;
 
 namespace syncer {
 namespace syncable {
 
 ostream& operator<<(ostream& out, const Id& id) {
   out << id.s_;
   return out;
 }
 
 base::StringValue* Id::ToValue() const {
   return new base::StringValue(s_);
 }
 
 string Id::GetServerId() const {
   // Currently root is the string "0". We need to decide on a true value.
   // "" would be convenient here, as the IsRoot call would not be needed.
+  DCHECK(!IsNull());
   if (IsRoot())
     return "0";
   return s_.substr(1);
 }
 
 Id Id::CreateFromServerId(const string& server_id) {
   Id id;
   if (server_id == "0")
     id.s_ = "r";
   else
     id.s_ = string("s") + server_id;
   return id;
 }
 
 Id Id::CreateFromClientString(const string& local_id) {
   Id id;
   if (local_id == "0")
     id.s_ = "r";
   else
     id.s_ = string("c") + local_id;
   return id;
 }
 
+Id Id::GetRoot() {
+  Id id;
+  id.s_ = "r";
+  return id;
+}
+
 Id Id::GetLexicographicSuccessor() const {
   // The successor of a string is given by appending the least
   // character in the alphabet.
   Id id = *this;
   id.s_.push_back(0);
   return id;
 }
 
 // static
 Id Id::GetLeastIdForLexicographicComparison() {
   Id id;
   id.s_.clear();
   return id;
 }
 
-Id GetNullId() {
-  return Id();  // Currently == root.
-}
-
 }  // namespace syncable
 }  // namespace syncer