Convert GN visibility variables to lists.

ui/keyboard/webui/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("//mojo/public/tools/bindings/mojom.gni")
 
 # GYP version: ui/keyboard/keyboard.gyp:keyboard_mojom_bindings
 mojom("mojo_bindings") {
   sources = [
     "keyboard.mojom",
   ]
 }
 
 # GYP version: part of ui/keyboard/keyboard.gyp:keyboard
 source_set("webui") {
-  visibility = "//ui/keyboard"
+  visibility = [ "//ui/keyboard" ]
 
   sources = [
     "../keyboard_export.h",
     "vk_mojo_handler.cc",
     "vk_mojo_handler.h",
     "vk_webui_controller.cc",
     "vk_webui_controller.h",
   ]
 
   deps = [
     ":mojo_bindings",
     "//base",
     "//content/public/browser",
     "//content/public/common",
     "//ui/aura",
     "//ui/base",
     "//ui/keyboard:resources",
     "//mojo/public/cpp/bindings",
     "//mojo/environment:chromium",
     "//mojo/bindings/js",
     "//mojo/system",
   ]
 
   defines = [ "KEYBOARD_IMPLEMENTATION" ]
 
 }