Add Mojo constant for "ash" and replace hardcoded strings

mash/simple_wm/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("//services/service_manager/public/cpp/service.gni")
 import("//services/service_manager/public/service_manifest.gni")
 import("//tools/grit/repack.gni")
 
 service("simple_wm") {
@@ skipping 19 matching lines @@
     "move_event_handler.cc",
     "move_event_handler.h",
     "move_loop.cc",
     "move_loop.h",
     "simple_wm.cc",
     "simple_wm.h",
   ]
 
   deps = [
     "//base",
+    "//mash/simple_wm/public/interfaces",
     "//services/service_manager/public/cpp",
     "//services/ui/public/cpp",
     "//services/ui/public/interfaces",
     "//ui/aura",
     "//ui/aura:test_support",
     "//ui/display",
     "//ui/display:test_support",
     "//ui/gfx/geometry/mojo",
     "//ui/views",
     "//ui/views/mus:mus",
     "//ui/wm",
   ]
 
   public_deps = [
     "//skia",  # Due to use of Sk types in service header.
   ]
 
   data_deps = [
     "//services/ui",
   ]
 }
 
 service_manifest("manifest") {
   name = "simple_wm"
   source = "manifest.json"
 }