[Sync] Move //sync to //components/sync.

sync/protocol/unique_position.proto

-// 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.
-//
-// Protobuf representation of the UniquePosition class.
-
-// Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
-// any fields in this file.
-
-syntax = "proto2";
-
-option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
-
-package sync_pb;
-
-// A UniquePosition is a string of bytes.
-//
-// Unique positions are unique per-item, since they are guaranteed to end with a
-// fixed-length suffix that is unique per-item.  The position string may not end
-// with a '\0' byte.
-//
-// Prior to the suffix is a series of arbitrary bytes of arbitrary length.
-// Items under the same parent are positioned relative to each other by a
-// lexicographic comparison of their UniquePosition values.
-message UniquePosition {
-  // History:
-  //
-  // Unique positions were first introduced in M28.  This change was rolled out
-  // in such a way that it would try to maintain backwards compatibilty with
-  // clients that understood only the old int64-based positions.
-  //
-  // At first, clients supported only the 'value' field.  This version never
-  // made it to stable.  We later added support for the 'compressed_value'
-  // field, and clients would populate either one or the other.
-  //
-  // In M30, we added the custom_compressed_v1 representation.  This
-  // representation was better than the previous implementations in almost every
-  // way.  However, we could not use it right away, since older clients would
-  // not understand it.  We decided to write both the old-style ('value' or
-  // 'custom_compressed') representation and the 'custom_compressed_v1'
-  // repersentations to every protobuf during the transition period.  Protobufs
-  // written during this transition period would be readable by clients who
-  // understand at least one of the two formats.
-  //
-  // In M33, we dropped support for writing the backwards-compatibility fields.
-  // Protobufs written by this version or later are not be intelligible by
-  // clients with version M29 or older.  Those clients will end up making use of
-  // the old int64 position fallback mechanism.
-
-  // The uncompressed string of bytes representing the position.
-  //
-  // Deprecated.  See history note above.
-  optional bytes value = 1;
-
-  // The client may choose to write a compressed position to this field instead
-  // of populating the 'value' above.  If it chooses to use compression, the
-  // 'value' field above must be empty.  The position value will be compressed
-  // with gzip and stored in the compressed_value field.  The position's
-  // uncompressed length must be specified and written to the
-  // uncompressed_length field.
-  //
-  // Deprecated.  See history note above.
-  optional bytes compressed_value = 2;
-  optional uint64 uncompressed_length = 3;
-
-  // This encoding uses compression scheme designed especially for unique
-  // positions.  It has the property that X < Y precisely when Compressed(X) <
-  // Compressed(Y), which is very useful when the most common operation is to
-  // compare these positions against each other.  Their values may remain
-  // compressed in memory.
-  //
-  // The compression scheme is implemented and documented in
-  // sync/internal_api/base/unique_position.cc.
-  //
-  // As of M30, this is the preferred encoding.  Newer clients may continue to
-  // populate the 'value' and 'compressed_value' fields to ensure backwards
-  // compatibility, but they will always try to read from this field first.
-  optional bytes custom_compressed_v1 = 4;
-}