Add Mojo application test support to Chromium.

mojo/application/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.
 
 # GYP version: mojo/mojo_base.gyp:mojo_application_chromium
 source_set("application") {
   sources = [
     "application_runner_chromium.cc",
     "application_runner_chromium.h",
   ]
 
   public_deps = [
     "//mojo/public/cpp/application",
   ]
   deps = [
     "//base",
     "//mojo/common",
     "//mojo/environment:chromium",
   ]
 }
+
+source_set("test_support") {
+  testonly = true
+  sources = [ "application_test_main_chromium.cc" ]
+
+  public_deps = [ "//mojo/public/cpp/application:test_support" ]
+  deps = [ "//base" ]
+}