Add Mojo constant for "ash" and replace hardcoded strings

mash/common/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")
+
 source_set("common") {
   sources = [
     "config.cc",
     "config.h",
   ]
 
   deps = [
     "//base",
   ]
+
+  if (use_ash) {
+    deps += [ "//ash/public/interfaces" ]
+  }
 }