Remove ash_unittests/mash_unittests from Windows build targets

ash/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.
 
+assert(is_chromeos)
+
 source_set("unittests") {
   testonly = true
   sources = [
     "accelerators/accelerator_table_unittest.cc",
     "system/date/date_view_unittest.cc",
     "system/update/tray_update_unittest.cc",
     "wm/container_finder_unittest.cc",
     "wm/mru_window_tracker_unittest.cc",
     "wm/workspace/workspace_layout_manager_unittest.cc",
     "wm_window_unittest.cc",
@@ skipping 16 matching lines @@
     sources += [
       "system/chromeos/audio/tray_audio_unittest.cc",
       "system/chromeos/brightness/tray_brightness_unittest.cc",
       "system/chromeos/network/vpn_list_unittest.cc",
       "system/chromeos/screen_security/screen_tray_item_unittest.cc",
       "system/chromeos/supervised/tray_supervised_user_unittest.cc",
     ]
     deps += [ "//ui/message_center" ]
   }
 }