Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/favicon_tracking_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 the favicon tracking type.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 // Tracking info for of favicon images. These control expiration of images
 // from sync based on recency, bookmark state, etc.
 message FaviconTrackingSpecifics {
   // The url of the favicon image.
   optional string favicon_url = 1;
   // The last time a page using this favicon was visited (in milliseconds
   // since linux epoch).
   optional int64 last_visit_time_ms = 3;
   // Whether this favicon is currently bookmarked or not.
   optional bool is_bookmarked = 4;
 }