Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/favicon_image_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 image specifics.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 message FaviconData {
   // The image data for the favicon (PNG encoded).
   optional bytes favicon = 1;
   // The favicon dimensions.
   optional int32 width = 2;
   optional int32 height = 3;
 }
@@ skipping 10 matching lines @@
 
   // A normal low-resolution web favicon (max resolution 16x16).
   optional FaviconData favicon_web = 2;
   // A slightly higher-resolution web favicon (max resolution 32x32).
   optional FaviconData favicon_web_32 = 3;
   // A high-resolution touch favicon (not precomposed, max resolution 64x64).
   optional FaviconData favicon_touch_64 = 4;
   // A high-resolution touch favicon (precomposed, max resolution 64x64).
   optional FaviconData favicon_touch_precomposed_64 = 5;
 }