Introduce mash/shell

mash/example/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("//testing/test.gni")
 
 group("example") {
   testonly = true
 
   deps = [
     "//mash/example/main",
-    "//mash/example/mock_sysui",
     "//mash/example/views_examples",
     "//mash/example/window_type_launcher",
     "//mash/wm",
     "//mojo/runner",
   ]
 }