Content "gn check" work.

content/public/common/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/buildflag_header.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
 import("//content/common/common.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//third_party/webrtc/build/webrtc.gni")
@@ skipping 60 matching lines @@
   ]
   sources = [
     "//content/common/content_export.h",
     "content_switches.cc",
   ]
 
   public_configs = [ ":static_switches_defines" ]
 }
 
 source_set("common_sources") {
+  # External code should depend on via ":common" above.
   visibility = [ "//content/*" ]
 
   sources = rebase_path(content_common_gypi_values.public_common_sources,
                         ".",
                         "//content")
 
   configs += [
     "//build/config:precompiled_headers",
     "//content:content_implementation",
   ]
@@ skipping 59 matching lines @@
 source_set("feature_h264_with_openh264_ffmpeg") {
   deps = [
     ":features",
     "//base",
   ]
   sources = [
     "feature_h264_with_openh264_ffmpeg.cc",
     "feature_h264_with_openh264_ffmpeg.h",
   ]
 }