Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/core_impl/attachments/proto/attachment_store.proto

 // Copyright 2014 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 attachment_store_pb;
 
 // Metadata for leveldb attachment store database.
 message StoreMetadata {
   // |schema_version| indicates format in which data is written in attachment
   // store. Needed for upgrade and to prevent newer data from being loaded by
   // older code that doesn't understand it.
   optional int32 schema_version = 1;
 }
@@ skipping 10 matching lines @@
 
   // Component enum mirrors values of AttachmentStore::Component.
   enum Component {
     UNKNOWN = 0;
     MODEL_TYPE = 1;
     SYNC = 2;
   }
   // Set of components that reference this attachment.
   repeated Component component = 3;
 }