Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/search_engine_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 custom search engines.
 
 // 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 custom search engine sync objects.
 message SearchEngineSpecifics {
   // The description of the search engine.
   optional string short_name = 1;
   // The search engine keyword for omnibox access.
   optional string keyword = 2;
   // A URL to the favicon to show in the search engines options page.
@@ skipping 48 matching lines @@
   // The following post_params are comma-separated lists used to specify the
   // post parameters for the corresponding search URL.
   optional string search_url_post_params = 22;
   optional string suggestions_url_post_params = 23;
   optional string instant_url_post_params = 24;
   optional string image_url_post_params = 25;
 
   // The parameterized URL for a search provider specified new tab page.
   optional string new_tab_url = 26;
 }