Makes AshTestImpl::Create create the right impl based on config

ash/test/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")
 
-# Used to test ash with an aura backend.
-source_set("ash_with_aura_test_support") {
-  testonly = true
-  sources = [
-    "ash_test_impl_aura.cc",
-    "ash_test_impl_aura.h",
-  ]
-  deps = [
-    ":test_support_common",
-    "//ash",
-    "//base",

[msw, 2017/03/31 16:27:13]

optional nit: move this generic //base dep to test_support_common?

[sky, 2017/03/31 17:37:22]

Done.

-    "//skia",
-    "//ui/aura:test_support",
-    "//ui/display",
-    "//ui/wm",
-  ]
-}
-
 static_library("test_support_without_content") {
   testonly = true
   sources = [
     "ash_test_environment_default.cc",
   ]
 
   public_deps = [
     ":test_support_common",
   ]
 
@@ skipping 48 matching lines @@
     "../common/test/test_system_tray_delegate.cc",
     "../common/test/test_system_tray_delegate.h",
     "../common/test/workspace_event_handler_test_helper.cc",
     "../common/test/workspace_event_handler_test_helper.h",
 
     # TODO(jamescook): Move these files into ash/test.
     "../laser/laser_pointer_controller_test_api.cc",
     "../laser/laser_pointer_controller_test_api.h",
     "../laser/laser_pointer_points_test_api.cc",
     "../laser/laser_pointer_points_test_api.h",
+    "../mus/test/ash_test_impl_mus.cc",
+    "../mus/test/ash_test_impl_mus.h",
+    "../mus/test/wm_test_base.cc",
+    "../mus/test/wm_test_base.h",
+    "../mus/test/wm_test_helper.cc",
+    "../mus/test/wm_test_helper.h",
     "../rotator/test/screen_rotation_animator_test_api.cc",
     "../rotator/test/screen_rotation_animator_test_api.h",
     "../shell/toplevel_window.cc",
     "../shell/toplevel_window.h",
     "ash_test_base.cc",
     "ash_test_base.h",
     "ash_test_environment.h",
     "ash_test_helper.cc",
     "ash_test_helper.h",
+    "ash_test_impl_aura.cc",
+    "ash_test_impl_aura.h",
     "ash_test_views_delegate.cc",
     "ash_test_views_delegate.h",
     "child_modal_window.cc",
     "child_modal_window.h",
     "cursor_manager_test_api.cc",
     "cursor_manager_test_api.h",
     "display_configuration_controller_test_api.cc",
     "display_configuration_controller_test_api.h",
     "immersive_fullscreen_controller_test_api.cc",
     "immersive_fullscreen_controller_test_api.h",
@@ skipping 107 matching lines @@
   deps = [
     ":test_support_with_content",
     "//base",
     "//skia",
     "//testing/gtest",
     "//ui/aura",
     "//ui/base",
     "//ui/gl:test_support",
   ]
 }