chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/common/BUILD.gn

Index: ash/common/BUILD.gn
diff --git a/ash/common/BUILD.gn b/ash/common/BUILD.gn
deleted file mode 100644
index 80c98b052037f29ed8c211446ab88907b8780c53..0000000000000000000000000000000000000000
--- a/ash/common/BUILD.gn
+++ /dev/null
@@ -1,40 +0,0 @@
-# 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",
-    "drag_drop/drag_image_view_unittest.cc",
-    "mus_property_mirror_ash_unittest.cc",
-    "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",
-    "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",
-    "wm_window_user_data_unittest.cc",
-  ]
-  deps = [
-    "//ash",
-    "//ash/common/test:test_support",
-    "//ash/public/cpp:ash_public_cpp",
-    "//ash/public/interfaces",
-    "//ash/test:test_support_without_content",
-    "//ui/aura",
-    "//ui/base",
-    "//ui/display",
-    "//ui/keyboard",
-    "//ui/message_center",
-    "//ui/views",
-    "//ui/wm",
-  ]
-}