Remove #pragma once from sync

sync/syncable/syncable_changes_version.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_SYNCABLE_SYNCABLE_CHANGES_VERSION_H_
 #define SYNC_SYNCABLE_SYNCABLE_CHANGES_VERSION_H_
-#pragma once
 
 namespace syncer {
 namespace syncable {
 
 // For the most part, the sync engine treats version numbers as opaque values.
 // However, there are parts of our code base that break this abstraction, and
 // depend on the following two invariants:
 //
 //  1.  CHANGES_VERSION is less than 0.
 //  2.  The server only issues positive version numbers.
 //
 // Breaking these abstractions makes some operations 10 times
 // faster.  If either of these invariants change, then those queries
 // must be revisited.
 
 enum {
   CHANGES_VERSION = -1
 };
 
 #define CHANGES_VERSION_STRING "-1"
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_SYNCABLE_CHANGES_VERSION_H_