Add mus_demo mojo app.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 593 matching lines @@
     deps += [ "//mojo/common:mojo_common_perftests" ]
   }
 
   if (!is_android && !is_ios && !is_chromeos) {
     deps += [ "//components/proximity_auth:proximity_auth_unittests" ]
   }
 
   if (is_win || is_linux) {
     deps += [
       ":mojo_apptests",
+      "//components/mus/demo",
       "//components/mus/ws:tests",
       "//mash:all",
       "//mojo",
     ]
   }
 
   if (is_win && use_drfuzz) {
     # build libfuzzer fuzzers on Windows to run with Dr. Fuzz
     deps += [
       "//testing/libfuzzer/fuzzers",
@@ skipping 276 matching lines @@
   assert(target_name != "")  # Mark as used.
   sources = invoker.actual_sources
   assert(
       sources == invoker.actual_sources,
       "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
 }
 
 assert_valid_out_dir("_unused") {
   actual_sources = [ "$root_build_dir/foo" ]
 }