Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/device_info_specifics.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.
 //
 // Sync protocol datatype extension for nigori keys.
 
 // 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;
 
 import "sync_enums.proto";
 
 // Information about a device that is running a sync-enabled Chrome browser.  We
 // are mapping the per-client cache guid to more specific information about the
 // device.
 message DeviceInfoSpecifics {
   // The cache_guid created to identify a sync client on this device.
@@ skipping 19 matching lines @@
   // Device_id that is stable until user signs out. This device_id is used for
   // annotating login scoped refresh token.
   optional string signin_scoped_device_id = 7;
 
   // This field is updated to be the current time periodically, and is also set
   // to the current time whenever another field changes. By examining the
   // difference between this field and the current time, it should be possible
   // to reason about the inactivity of any device that was syncing at one time.
   optional int64 last_updated_timestamp = 8;
 }