Content "gn check" work.

content/public/plugin/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.
 
 # See //content/BUILD.gn for how this works.
 group("plugin") {
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
       ":plugin_sources",
     ]
   }
 }
 
 source_set("plugin_sources") {
+  # External code should depend on via ":plugin" above.
   visibility = [ "//content/*" ]
 
   sources = [
     "content_plugin_client.h",
   ]
 
   deps = [
     "//base",
-    "//content/plugin",
+    "//content/plugin",  # Must not be a public dep.
     "//content/public/common:common_sources",
   ]
+
+  allow_circular_includes_from = [
+    # This target is a pair with content/plugin. They always go together and
+    # include headers from each other.
+    "//content/plugin",
+  ]
 }