Add simple mash context menu support.

ash/mus/BUILD.gn

 # Copyright 2016 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("//mojo/public/mojo_application.gni")
 import("//mojo/public/mojo_application_manifest.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//tools/grit/repack.gni")
 
 source_set("lib") {
   sources = [
+    "context_menu_mus.cc",
+    "context_menu_mus.h",
     "keyboard_ui_mus.cc",
     "keyboard_ui_mus.h",
     "shelf_delegate_mus.cc",
     "shelf_delegate_mus.h",
     "shell_delegate_mus.cc",
     "shell_delegate_mus.h",
     "stub_context_factory.cc",
     "stub_context_factory.h",
     "sysui_application.cc",
     "sysui_application.h",
@@ skipping 54 matching lines @@
   deps = [
     ":lib",
     "//mojo/shell/public/cpp",
   ]
 
   data_deps = [
     ":manifest",
     "//components/mus",
   ]
 }