Make more of content pass "gn check"

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")
 
 # See //content/BUILD.gn for how this works.
 group("child") {
   if (is_component_build) {
     public_deps = [
@@ skipping 20 matching lines @@
                         "//content")
 
   configs += [ "//content:content_implementation" ]
 
   public_deps = [
     "//content/child",
   ]
 
   deps = [
     "//content/public/common:common_sources",
+    "//gin",
   ]
 }