Add missing dependencies to the GN build.

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 = [ "//content" ]
   } else {
     public_deps = [ ":child_sources" ]
   }
 }
 
 source_set("child_sources") {
   visibility = [ "//content/*" ]
 
   sources = rebase_path(content_child_gypi_values.public_child_sources,
                         ".", "//content")
 
   configs += [
     "//content:content_implementation",
   ]
 
+  public_deps = [
+    "//content/child",
+  ]
+
   deps = [
-    "//content/child",
     "//content/public/common:common_sources",
   ]
 }