Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/theme_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 themes.
 
 // 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;
 
 // Properties of theme sync objects.
 message ThemeSpecifics {
   // If set, we're using a custom theme and all custom_* fields should be
   // present.  If not set, we use the default or system theme (see below)
   // and all custom_* fields should be omitted.
   optional bool use_custom_theme = 1;
   // This field is only set (i.e., not cleared) on platforms that have
   // a distinction between the system theme and the default theme, but
   // other platforms must be careful to pass through the set state (not
   // just the value) of this flag.
   //
   // If true, we use the system theme by default (i.e., when we don't use
   // a custom theme) for platforms that make a distinction between the
   // default theme and the system theme.  Has no effect if use_custom_theme
   // is set.
   optional bool use_system_theme_by_default = 2;
 
   // Custom-theme-specific fields.
   optional string custom_theme_name = 3;
   optional string custom_theme_id = 4;
   optional string custom_theme_update_url = 5;
 }