Set enable_hangout_services_extension from branding.

chrome/common/features.gni

Index: chrome/common/features.gni
diff --git a/chrome/common/features.gni b/chrome/common/features.gni
index cfd943792bc0ceb428ed7082cc5f6847b73e0cd4..5bf51d569eb7a37d0dc340e839a584faf6f1b10f 100644
--- a/chrome/common/features.gni
+++ b/chrome/common/features.gni
@@ -29,8 +29,8 @@ declare_args() {
   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
@@ -68,6 +68,7 @@ declare_args() {
 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",