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

ash/BUILD.gn

Index: ash/BUILD.gn
diff --git a/ash/BUILD.gn b/ash/BUILD.gn
index 5f3c85c8412fb3ce2d1965e8ca1a73a0d90ea28d..03bb0b9a91a1548773ef6be6be9a65cd4d763721 100644
--- a/ash/BUILD.gn
+++ b/ash/BUILD.gn
@@ -1092,7 +1092,6 @@ test("ash_content_unittests") {
     ":ash",
     ":ash_with_content",
     "//ash/public/cpp:ash_public_cpp",
-    "//ash/test:ash_with_aura_test_support",
     "//ash/test:test_support_with_content",
     "//base",
     "//base/test:test_support",
@@ -1409,7 +1408,6 @@ test("ash_unittests") {
     "//ash/resources",
     "//ash/resources/vector_icons",
     "//ash/strings",
-    "//ash/test:ash_with_aura_test_support",
     "//ash/test:test_support_without_content",
     "//ash/touch_hud",
     "//base",