Add enable_hangout_services_extension flag to GN build.

build/secondary/tools/grit/grit_rule.gni

Index: build/secondary/tools/grit/grit_rule.gni
diff --git a/build/secondary/tools/grit/grit_rule.gni b/build/secondary/tools/grit/grit_rule.gni
index 8726927e57990e8f1a167a6fc2b775036a0bc862..d49c4ed55022171c504d8a1e7bb036ddf9b7a600 100644
--- a/build/secondary/tools/grit/grit_rule.gni
+++ b/build/secondary/tools/grit/grit_rule.gni
@@ -182,8 +182,9 @@ if (enable_google_now) {
 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" ]
 }