Content "gn check" work.

content/public/gpu/BUILD.gn

 # Copyright 2016 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.
 
 # See //content/BUILD.gn for how this works.
 group("gpu") {
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
       ":gpu_sources",
     ]
   }
 }
 
 source_set("gpu_sources") {
+  # You must depend on this via //content/public/gpu above for the component
+  # build to work properly.
   visibility = [ "//content/*" ]
 
   configs += [ "//content:content_implementation" ]
 
   sources = [
     "content_gpu_client.h",
   ]
 
   deps = [
     "//base",
     "//content:export",
     "//content/gpu:gpu_sources",
     "//content/public/common:common_sources",
   ]
+
+  allow_circular_includes_from = [ "//content/gpu:gpu_sources" ]
 }