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

ash/test/ash_test_helper.h

 // Copyright 2013 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.
 
 #ifndef ASH_TEST_ASH_TEST_HELPER_H_
 #define ASH_TEST_ASH_TEST_HELPER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 47 matching lines @@
 class TestSessionControllerClient;
 class TestSessionStateDelegate;
 
 // A helper class that does common initialization required for Ash. Creates a
 // root window and an ash::Shell instance with a test delegate.
 class AshTestHelper {
  public:
   explicit AshTestHelper(AshTestEnvironment* ash_test_environment);
   ~AshTestHelper();
 
+  // Returns the configuration tests are run in.

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

nit: "that tests", optionally mention mus to clarify the generic terminology?

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

Done.

+  static Config config() { return config_; }
+
   // Creates the ash::Shell and performs associated initialization.  Set
   // |start_session| to true if the user should log in before the test is run.
   void SetUp(bool start_session);
 
   // Destroys the ash::Shell and performs associated cleanup.
   void TearDown();
 
   // Returns a root Window. Usually this is the active root Window, but that
   // method can return NULL sometimes, and in those cases, we fall back on the
   // primary root Window.
@@ skipping 81 matching lines @@
 
   std::unique_ptr<TestSessionControllerClient> session_controller_client_;
 
   DISALLOW_COPY_AND_ASSIGN(AshTestHelper);
 };
 
 }  // namespace test
 }  // namespace ash
 
 #endif  // ASH_TEST_ASH_TEST_HELPER_H_