[Sync] Move //sync to //components/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_
-
-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_