Make chrome GN build work in component mode.

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")
 
-source_set("child") {
+# See //content/BUILD.gn for how this works.
+group("child") {
+  if (is_component_build) {
+    deps = [ "//content" ]
+  } else {
+    deps = [ ":child_sources" ]
+  }
+  forward_dependent_configs_from = deps
+}
+
+source_set("child_sources") {
+  visibility = [ "//content/*" ]
+
   sources = rebase_path(content_child_gypi_values.public_child_sources,
                         ".", "//content")
 
   configs += [
     "//content:content_implementation",
   ]
 
   deps = [
     "//content/child",
-    "//content/public/common",
+    "//content/public/common:common_sources",
   ]
 }