Work on "gn check" of content/*

content/public/child/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("//content/child/child.gni")
 
 source_set("child") {
   sources = rebase_path(content_child_gypi_values.public_child_sources,
                         ".", "//content")
 
   configs += [
     "//content:content_implementation",
   ]
 
   deps = [
     "//content/child",
+    "//content/public/common",
   ]
 }