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

ash/test/ash_test_helper.h

Index: ash/test/ash_test_helper.h
diff --git a/ash/test/ash_test_helper.h b/ash/test/ash_test_helper.h
index 8d419477de6e6cc3d0412c94a6d2d3f27ef667d7..d016ebf1930f8620b9ac8d8cad65270ee315f64f 100644
--- a/ash/test/ash_test_helper.h
+++ b/ash/test/ash_test_helper.h
@@ -65,6 +65,10 @@ class AshTestHelper {
   explicit AshTestHelper(AshTestEnvironment* ash_test_environment);
   ~AshTestHelper();
 
+  // Returns the configuration that tests are run in. See ash::Config enum for
+  // details.
+  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);