Content "gn check" work.

content/public/renderer/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("//build/config/features.gni")
 import("//content/renderer/renderer.gni")
 
 # See //content/BUILD.gn for how this works.
 group("renderer") {
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
       ":renderer_sources",
     ]
   }
 }
 
 source_set("renderer_sources") {
+  # External code should depend on via ":renderer" above.
   visibility = [ "//content/*" ]
 
   sources = rebase_path(content_renderer_gypi_values.public_renderer_sources,
                         ".",
                         "//content")
 
   configs += [ "//content:content_implementation" ]
 
   deps = [
     "//content/public/child:child_sources",
@@ skipping 31 matching lines @@
     ]
   }
 
   if (enable_plugins) {
     sources +=
         rebase_path(content_renderer_gypi_values.public_renderer_plugin_sources,
                     ".",
                     "//content")
   }
 }