Add enable_hangout_services_extension flag to GN build.

build/secondary/tools/grit/grit_rule.gni

 # 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.
 
 # Instantiate grit. This will produce a script target to run grit, and a
 # static library that compiles the .cc files.
 #
 # Parameters
 #
 #   source (required)
@@ skipping 164 matching lines @@
   grit_defines += [ "-D", "enable_settings_app" ]
 }
 if (enable_google_now) {
   grit_defines += [ "-D", "enable_google_now" ]
 }
 # Note: use_concatenated_impulse_responses is omitted. It is never used and
 # should probably be removed from GYP build.
 if (enable_webrtc) {
   grit_defines += [ "-D", "enable_webrtc" ]
 }
-# Note: enable_hangout_services_extension is omitted. It is never set in the
-# GYP build. Need to figure out what it's for.
+if (enable_hangout_services_extension) {
+  grit_defines += [ "-D", "enable_hangout_services_extension" ]
+}
 if (enable_task_manager) {
   grit_defines += [ "-D", "enable_task_manager" ]
 }
 if (enable_notifications) {
   grit_defines += [ "-D", "enable_notifications" ]
 }
 if (enable_wifi_bootstrapping) {
   grit_defines += [ "-D", "enable_wifi_bootstrapping" ]
 }
 if (enable_service_discovery) {
@@ skipping 136 matching lines @@
     if (defined(invoker.public_configs)) {
       public_configs += invoker.public_configs
     }
 
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
     output_name = grit_output_name
   }
 }