Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/entity_metadata.proto

 // Copyright 2015 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 // Sync proto to store entity metadata in model type storage.
 message EntityMetadata {
   // A hash based on the client tag and model type.
   // Used for various map lookups. Should always be available.
   // Sent to the server as SyncEntity::client_defined_unique_tag.
   optional string client_tag_hash = 1;
 
@@ skipping 33 matching lines @@
   // entity's specifics value has changed without having to keep specifics in
   // memory.
   optional string specifics_hash = 9;
 
   // A hash of the last specifics known by both the client and server. Used to
   // detect when local commits and remote updates are just for encryption. This
   // value will be the empty string only in the following cases: the entity is
   // in sync with the server, has never been synced, or is deleted.
   optional string base_specifics_hash = 10;
 }