Reland: mus: Enable system modal windows

components/mus/ws/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("//build/config/ui.gni")
 import("//testing/test.gni")
 import("//mojo/public/mojo_application.gni")
 import("//mojo/public/mojo_application_manifest.gni")
 
 source_set("lib") {
@@ skipping 13 matching lines @@
     "display_manager_delegate.h",
     "event_dispatcher.cc",
     "event_dispatcher.h",
     "event_dispatcher_delegate.h",
     "event_matcher.cc",
     "event_matcher.h",
     "focus_controller.cc",
     "focus_controller.h",
     "focus_controller_delegate.h",
     "focus_controller_observer.h",
+    "modal_window_controller.cc",
+    "modal_window_controller.h",
     "operation.cc",
     "operation.h",
     "platform_display.cc",
     "platform_display.h",
     "platform_display_delegate.h",
     "platform_display_factory.h",
     "platform_display_init_params.cc",
     "platform_display_init_params.h",
     "platform_screen.h",
     "platform_screen_impl.cc",
@@ skipping 195 matching lines @@
 
   data_deps = [
     ":mus_ws_unittests_app_manifest",
   ]
 }
 
 mojo_application_manifest("mus_ws_unittests_app_manifest") {
   application_name = "mus_ws_unittests_app"
   source = "mus_ws_unittests_app_manifest.json"
 }