Adds TODO with reference to bug

ash/mus/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/tools/bindings/mojom.gni")
 import("//services/shell/public/cpp/service.gni")
 import("//services/shell/public/service_manifest.gni")
 import("//tools/grit/repack.gni")
 
@@ skipping 199 matching lines @@
     "//ui/gfx/geometry",
     "//ui/gfx/geometry/mojo",
     "//ui/message_center",
     "//ui/views/mus",
   ]
 
   data_deps = [
     ":mus",
     "//mash/quick_launch",
   ]
+
+  defines = [ "NOTIMPLEMENTED_POLICY=5" ]
 }