Add content/child 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/common/common.gni")
+import("//content/child/child.gni")
 
-source_set("common") {
-  sources = rebase_path(content_common_gypi_values.public_common_sources,
+source_set("child") {
+  sources = rebase_path(content_child_gypi_values.public_child_sources,
                         ".", "//content")
 
   configs += [
     "//content:content_implementation",
   ]
 
   deps = [
-    "//content/common",
-    "//skia",
-    "//third_party/WebKit/public:blink_headers",
+    "//content/child",
   ]
 }