Content "gn check" work.

content/ppapi_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.
 
 group("ppapi_plugin") {
-  visibility = [ "//content/*" ]
+  visibility = [ "//content/*" ]  # This is an internal content API.
 
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
       ":ppapi_plugin_sources",
     ]
   }
 }
 
 source_set("ppapi_plugin_sources") {
-  visibility = [ "//content/*" ]
+  # Depend on via ":ppapi_plugin above.
+  visibility = [
+    ":ppapi_plugin",
+    "//content",  # For the component build.
+  ]
 
   sources = [
     "broker_process_dispatcher.cc",
     "broker_process_dispatcher.h",
     "plugin_process_dispatcher.cc",
     "plugin_process_dispatcher.h",
     "ppapi_blink_platform_impl.cc",
     "ppapi_blink_platform_impl.h",
     "ppapi_broker_main.cc",
     "ppapi_plugin_main.cc",
@@ skipping 11 matching lines @@
     "//mojo/shell/public/interfaces",
     "//ppapi/proxy:ipc",
     "//skia",
     "//third_party/WebKit/public:blink",
     "//third_party/icu",
     "//ui/base",
     "//ui/gfx",
     "//ui/gfx/geometry",
   ]
 }