GN: Add assert that ash/BUILD.gn referenced only on supported platforms

ash/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("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
+assert(use_ash)
+
 gypi_values = exec_script("//build/gypi_to_gn.py",
                           [ rebase_path("ash.gyp") ],
                           "scope",
                           [ "ash.gyp" ])
 
 component("ash") {
   sources = gypi_values.ash_sources
 
   configs += [ "//build/config:precompiled_headers" ]
   defines = [ "ASH_IMPLEMENTATION" ]
@@ skipping 385 matching lines @@
     configs += [ "//build/config/win:windowed" ]
     deps += [ "//sandbox" ]
   }
 
   if (is_chromeos) {
     deps += [ "//device/bluetooth" ]
   }
 }
 # When adding support for isolates, please have a look at run-time dependencies
 # in the ash_unittests_run target in ash.gyp.