Add enable_hangout_services_extension flag to GN build.

build/config/BUILD.gn

 # Copyright (c) 2013 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/allocator.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 
 declare_args() {
@@ skipping 186 matching lines @@
   }
   if (use_athena) {
     defines += [ "USE_ATHENA=1" ]
   }
   if (enable_hidpi) {
     defines += [ "ENABLE_HIDPI=1" ]
   }
   if (proprietary_codecs) {
     defines += [ "USE_PROPRIETARY_CODECS" ]
   }
+  if (enable_hangout_services_extension) {
+    defines += [ "ENABLE_HANGOUT_SERVICES_EXTENSION=1" ]
+  }
 }
 
 # Debug/release ----------------------------------------------------------------
 
 config("debug") {
   defines = [
     "_DEBUG",
     "DYNAMIC_ANNOTATIONS_ENABLED=1",
     "WTF_USE_DYNAMIC_ANNOTATIONS=1",
   ]
@@ skipping 86 matching lines @@
       "CoreText.framework",
       "Foundation.framework",
       "UIKit.framework",
     ]
   } else if (is_linux) {
     libs = [
       "dl",
     ]
   }
 }