Make chrome GN build work in component mode.

content/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("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//content/child/child.gni")
 
 source_set("child") {
-  # Only targets in the content tree can depend directly on this target.
-  visibility = [ "//content/*" ]
+  # Only the public target should depend on this. All other targets (even
+  # internal content ones) should depend on the public one.
+  visibility = [ "//content/public/child:child_sources" ]
 
   sources = rebase_path(content_child_gypi_values.private_child_sources,
                         ".", "//content")
 
   deps = [
     "//base",
     "//components/tracing",
     "//mojo/environment:chromium",
     "//mojo/public/interfaces/application",
     "//skia",
@@ skipping 99 matching lines @@
   }
 
   if (!use_openssl) {
     sources += rebase_path(content_child_gypi_values.webcrypto_nss_sources,
                            ".", "//content")
   } else {
     sources += rebase_path(content_child_gypi_values.webcrypto_openssl_sources,
                            ".", "//content")
   }
 }