Convert bindings targets in mojo/mojo_services.gypi to GN.

mojo/services/test_service/BUILD.gn

 # 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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
-# GYP version: chrome/chrome_web_ui_mojo_bindings.gyp:webui_mojo_bindings
-mojom("mojo_bindings") {
+# GYP version: mojo/mojo_services.gypi:mojo_test_service_bindings
+mojom("bindings") {
   sources = [
-    "omnibox.mojom",
+    "test_request_tracker.mojom",
+    "test_service.mojom",
   ]
 }