Enable Null Syncable ID which is different than Root ID.

sync/internal_api/sync_manager_impl_unittest.cc

Index: sync/internal_api/sync_manager_impl_unittest.cc
diff --git a/sync/internal_api/sync_manager_impl_unittest.cc b/sync/internal_api/sync_manager_impl_unittest.cc
index 0f58bb93ecea86c94bc9f28c0975f6fce7727d12..7bac38da59d3e9c447dd50e69720fd868c19ab68 100644
--- a/sync/internal_api/sync_manager_impl_unittest.cc
+++ b/sync/internal_api/sync_manager_impl_unittest.cc
@@ -155,7 +155,10 @@ int64 MakeServerNodeForType(UserShare* share,
   entry.PutBaseVersion(1);
   entry.PutServerVersion(1);
   entry.PutIsUnappliedUpdate(false);
-  entry.PutServerParentId(syncable::GetNullId());
+  // TODO(stanisc): setting parent ID might be unnecessary once
+  // empty parent ID is supported.
+  entry.PutParentId(syncable::Id::GetRoot());
+  entry.PutServerParentId(syncable::Id::GetRoot());
   entry.PutServerIsDir(true);
   entry.PutIsDir(true);
   entry.PutServerSpecifics(specifics);