ash/mash: Add a mus-client that sets up ash to provide the system ui for mash.

mash/shell/BUILD.gn

 # Copyright 2015 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")
 
@@ skipping 16 matching lines @@
     "//mojo/shell/public/cpp:sources",
   ]
 
   data_deps = [
     "//mash/browser_driver",
     "//mash/quick_launch",
     "//mash/shelf",
     "//mash/wallpaper",
     "//mash/wm",
   ]
+
+  if (use_ash) {
+    data_deps += [ "//ash/mus" ]
+  }
 }
 
 mojo_application_manifest("manifest") {
   application_name = "mash_shell"
   source = "manifest.json"
 }