Enable Null Syncable ID which is different than Root ID.

sync/syncable/entry_kernel.h

 // 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.
 
 #ifndef SYNC_SYNCABLE_ENTRY_KERNEL_H_
 #define SYNC_SYNCABLE_ENTRY_KERNEL_H_
 
 #include <set>
 
 #include "base/time/time.h"
@@ skipping 229 matching lines @@
   inline void put(Int64Field field, int64 value) {
     int64_fields[field - INT64_FIELDS_BEGIN] = value;
   }
   inline void put(TimeField field, const base::Time& value) {
     // Round-trip to proto time format and back so that we have
     // consistent time resolutions (ms).
     time_fields[field - TIME_FIELDS_BEGIN] =
         ProtoTimeToTime(TimeToProtoTime(value));
   }
   inline void put(IdField field, const Id& value) {
+    DCHECK(!value.IsNull());
     id_fields[field - ID_FIELDS_BEGIN] = value;
   }
   inline void put(BaseVersion field, int64 value) {
     int64_fields[field - INT64_FIELDS_BEGIN] = value;
   }
   inline void put(IndexedBitField field, bool value) {
     bit_fields[field - BIT_FIELDS_BEGIN] = value;
   }
   inline void put(IsDelField field, bool value) {
     bit_fields[field - BIT_FIELDS_BEGIN] = value;
@@ skipping 120 matching lines @@
     const EntryKernelMutation& mutation);
 
 // Caller owns the return value.
 base::ListValue* EntryKernelMutationMapToValue(
     const EntryKernelMutationMap& mutations);
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif // SYNC_SYNCABLE_ENTRY_KERNEL_H_