Convert GN visibility variables to lists.

media/cast/test/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")
 
 # GYP version: media/cast/cast.gyp:cast_logging_proto
 source_set("proto") {
   deps = [
     ":cast_network_simulation_proto",
   ]
   direct_dependent_configs = [
     "//third_party/protobuf:protobuf_config"
   ]
   forward_dependent_configs_from = [
     ":cast_network_simulation_proto",
   ]
 }
 
 proto_library("cast_network_simulation_proto") {
-  visibility = ":proto"
+  visibility = [ ":proto" ]
   sources = [
     "network_simulation_model.proto",
   ]
 }