Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/encryption.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.
 //
 // Common sync protocol for encrypted data.
 
 // 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;
 
 // Encrypted sync data consists of two parts: a key name and a blob. Key name is
 // the name of the key that was used to encrypt blob and blob is encrypted data
 // itself.
 //
 // The reason we need to keep track of the key name is that a sync user can
 // change their passphrase (and thus their encryption key) at any time. When
 // that happens, we make a best effort to reencrypt all nodes with the new
 // passphrase, but since we don't have transactions on the server-side, we
 // cannot guarantee that every node will be reencrypted. As a workaround, we
 // keep track of all keys, assign each key a name (by using that key to encrypt
 // a well known string) and keep track of which key was used to encrypt each
 // node.
 message EncryptedData {
   optional string key_name = 1;
   optional string blob = 2;
 };