Move ENABLE_HIDPI to new build flags system

ash/resources/BUILD.gn

 # Copyright 2014 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("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
+import("//ui/base/ui_features.gni")
 
 assert(enable_hidpi)
 
 grit("resources") {
   source = "ash_resources.grd"
   outputs = [
     "grit/ash_resources.h",
     "ash_resources_100_percent.pak",
     "ash_resources_200_percent.pak",
   ]
@@ skipping 45 matching lines @@
   }
 }
 
 ash_test_resources("100_percent") {
   percent = "100"
 }
 
 ash_test_resources("200_percent") {
   percent = "200"
 }