Enable Null Syncable ID which is different than Root ID.

sync/engine/syncer_util_unittest.cc

Index: sync/engine/syncer_util_unittest.cc
diff --git a/sync/engine/syncer_util_unittest.cc b/sync/engine/syncer_util_unittest.cc
index 9fda35f6bf76b12c7107c215b78e90faebc16784..27a59e4d2cdecc08d0cbf79d02f73a10f06f0f9a 100644
--- a/sync/engine/syncer_util_unittest.cc
+++ b/sync/engine/syncer_util_unittest.cc
@@ -163,7 +163,7 @@ sync_pb::EntitySpecifics DefaultBookmarkSpecifics() {
 TEST_F(GetUpdatePositionTest, UpdateServerFieldsFromUpdateTest) {
   InitSuffixIngredients();  // Initialize update with valid data.
 
-  std::string root_server_id = syncable::GetNullId().GetServerId();
+  std::string root_server_id = syncable::Id::GetRoot().GetServerId();
   int64 handle = entry_factory()->CreateUnappliedNewBookmarkItemWithParent(
       "I", DefaultBookmarkSpecifics(), root_server_id);
 
@@ -186,7 +186,7 @@ TEST_F(GetUpdatePositionTest, UpdateServerFieldsFromUpdateTest) {
 TEST_F(GetUpdatePositionTest, UpdateServerFieldsFromInvalidUpdateTest) {
   // Do not initialize data in update, update is invalid.
 
-  std::string root_server_id = syncable::GetNullId().GetServerId();
+  std::string root_server_id = syncable::Id::GetRoot().GetServerId();
   int64 handle = entry_factory()->CreateUnappliedNewBookmarkItemWithParent(
       "I", DefaultBookmarkSpecifics(), root_server_id);