Add content/child to the GN build.

content/child/child.gni

Index: content/child/child.gni
diff --git a/content/browser/browser.gni b/content/child/child.gni
similarity index 70%
copy from content/browser/browser.gni
copy to content/child/child.gni
index b7e8d7f1d52af468d5e4c8c1b7e34c69aba59029..a2ff699ea42d6d3fcdf1dfcb31895d52dafb92dc 100644
--- a/content/browser/browser.gni
+++ b/content/child/child.gni
@@ -2,14 +2,14 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# This file defines the content browser gypi values. This file is read once and
+# This file defines the content child gypi values. This file is read once and
 # cached, which is a performance optimization that allows us to share the
 # results of parsing the .gypi file between the public and private BUILD.gn
 # files. It also saves us from duplicating this exec_script call.
-content_browser_gypi_values = exec_script(
+content_child_gypi_values = exec_script(
     "//build/gypi_to_gn.py",
-    [ rebase_path("../content_browser.gypi"),
+    [ rebase_path("../content_child.gypi"),
       "--replace=<(SHARED_INTERMEDIATE_DIR)=$root_gen_dir" ],
     "scope",
-    [ "../content_browser.gypi" ])
+    [ "../content_child.gypi" ])