Convert GN visibility variables to lists.

content/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("//tools/grit/grit_rule.gni")
 
 # Applied by targets internal to content.
 config("content_implementation") {
   defines = [ "CONTENT_IMPLEMENTATION" ]
 }
@@ skipping 31 matching lines @@
   outputs = [
     "grit/content_resources.h",
     "content_resources.pak",
     "content_resources.rc",
   ]
 }
 
 # This target exists to "hold" the content_export header so we can do proper
 # inclusion testing of it.
 source_set("export") {
-  visibility = "//content/*"
+  visibility = [ "//content/*" ]
   sources = [
     "content/common/content_export.h"
   ]
 }