[Sync] Move sync/{internal_api,syncable} into syncer namespace

sync/test/null_transaction_observer.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.
 
 #ifndef SYNC_TEST_NULL_TRANSACTION_OBSERVER_H_
 #define SYNC_TEST_NULL_TRANSACTION_OBSERVER_H_
 #pragma once
 
 #include "sync/internal_api/public/util/weak_handle.h"
 
+namespace syncer {
 namespace syncable {
 
 class TransactionObserver;
 
 // Returns an initialized weak handle to a transaction observer that
 // does nothing.
 syncer::WeakHandle<TransactionObserver> NullTransactionObserver();
 
 }  // namespace syncable
+}  // namespace syncer
 
 #endif  // SYNC_TEST_NULL_TRANSACTION_OBSERVER_H_