Content "gn check" work.

content/public/child/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("//content/child/child.gni")
 
 # See //content/BUILD.gn for how this works.
 group("child") {
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
       ":child_sources",
     ]
     if (!is_android) {
       public_deps += [
         "//content/gpu",
         "//content/ppapi_plugin",
       ]
     }
   }
 }
 
 source_set("child_sources") {
+  # External code should depend in via ":child" above.
   visibility = [ "//content/*" ]
 
   sources = rebase_path(content_child_gypi_values.public_child_sources,
                         ".",
                         "//content")
 
   configs += [ "//content:content_implementation" ]
 
   public_deps = [
     "//content/child",
   ]
 
   deps = [
     "//content/public/common:common_sources",
     "//gin",
   ]
 }