Add content/child to the GN build.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 16 matching lines @@
     "//components/onc",
     "//components/os_crypt",
     "//components/query_parser",
     "//components/resources:components_resources",
     "//components/startup_metric_utils",
     "//components/tracing",
     "//components/translate:translate_core_browser",
     "//components/translate:translate_core_common",
     "//components/url_matcher",
     #"//content/public/browser",  # Exists but doesn't build yet.
+    "//content/public/child",
     "//content/public/common",
     "//content/public/renderer",
     "//crypto",
     "//device/usb",
     #"//extensions/common/api:extensions_api",
     "//gin",
     "//gpu",
     "//google_apis",
     "//ipc",
     "//mojo",
@@ skipping 72 matching lines @@
 
       # This stuff all depends on ui/surface which requires some .class jni
       # generators (ui/gl/gl.gyp:surface_jni_headers).
       "//ui/gl",
       "//gpu",
       "//ui/snapshot",
       "//ui/surface",
     ]
   }
 }