Windows GN component build fixes: sync, ui

sync/internal_api/attachments/proto/BUILD.gn

 # 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.
 
 import("//third_party/protobuf/proto_library.gni")
 
 proto_library("proto") {
+  # This should only get compiled into the sync component.
+  visibility = [ "//sync:sync_core" ]
+
   sources = [
     "attachment_store.proto",
   ]
   cc_generator_options = "dllexport_decl=SYNC_EXPORT_PRIVATE:"
   cc_include = "sync/base/sync_export.h"
 
   defines = [ "SYNC_IMPLEMENTATION" ]
 }