Set enable_hangout_services_extension from branding.

chrome/common/features.gni

 # Copyright 2015 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/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
 
 # Please keep features in alphabetical order.
@@ skipping 11 matching lines @@
   # and have a system dialog.
   enable_basic_print_dialog = enable_basic_printing && !is_chromeos
 
   enable_captive_portal_detection = !is_android && !is_ios && !is_chromecast
 
   # Google Now is disabled to prepare for its removal.
   # http://crbug.com/539674
   enable_google_now = false
 
   # Hangout services is an extension that adds extra features to Hangouts.
-  # For official Chrome builds, this flag is set on the bots.
-  enable_hangout_services_extension = false
+  # It is enableable separately to facilitate testing.
+  enable_hangout_services_extension = is_chrome_branded
 
   # 'Ok Google' hotwording is disabled by default. Set to true to enable. (This
   # will download a closed-source NaCl module at startup.) Chrome-branded
   # ChromeOS builds have this enabled by default.
   enable_hotwording = is_chrome_branded && is_chromeos
 
   enable_one_click_signin =
       is_win || is_mac || (is_linux && !is_chromeos && !is_chromecast)
 
   # Set to true to bundle all the mash related mojo services into chrome.
@@ skipping 17 matching lines @@
   pgo_build = chrome_pgo_phase > 0
 
   # Use vulcanized HTML/CSS/JS resources to speed up WebUI (chrome://)
   # pages. https://github.com/polymer/vulcanize
   use_vulcanize = true
 }
 
 chrome_grit_defines = [
   "enable_background=$enable_background",
   "enable_google_now=$enable_google_now",
+  "enable_hangout_services_extension=$enable_hangout_services_extension",
   "enable_hotwording=$enable_hotwording",
   "enable_service_discovery=$enable_service_discovery",
   "enable_vr_shell=$enable_vr_shell",
   "enable_webvr=$enable_webvr",
   "safe_browsing_mode=$safe_browsing_mode",
   "use_vulcanize=$use_vulcanize",
 ]