Add chrome/browser/extensions to "GN check"

ui/chromeos/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("//testing/test.gni")
 
-component("ui_chromeos") {
+assert(is_chromeos)
+
+component("chromeos") {
+  output_name = "ui_chromeos"
   sources = [
     "accelerometer/accelerometer_util.cc",
     "accelerometer/accelerometer_util.h",
     "ime/candidate_view.cc",
     "ime/candidate_view.h",
     "ime/candidate_window_view.cc",
     "ime/candidate_window_view.h",
     "ime/infolist_window.cc",
     "ime/infolist_window.h",
     "ime/input_method_menu_item.cc",
@@ skipping 55 matching lines @@
   sources = [
     "ime/candidate_view_unittest.cc",
     "ime/candidate_window_view_unittest.cc",
     "ime/input_method_menu_item_unittest.cc",
     "ime/input_method_menu_manager_unittest.cc",
     "network/network_state_notifier_unittest.cc",
     "run_all_unittests.cc",
     "touch_exploration_controller_unittest.cc",
   ]
   deps = [
-    ":ui_chromeos",
+    ":chromeos",
     "//base/test:test_support",
     "//chromeos",
     "//skia",
     "//testing/gtest",
     "//ui/aura:test_support",
     "//ui/compositor",
     "//ui/events:test_support",
     "//ui/gl:test_support",
     "//ui/message_center",
     "//ui/resources:ui_test_pak",
     "//ui/views",
     "//ui/views:test_support",
   ]
   data = [
     "$root_out_dir/locales/en-US.pak",
     "$root_out_dir/ui_test.pak",
   ]
 }