mash: Move directly linked NewWindowDelegate to mojom::NewWindowClient.

ash/mus/test/wm_test_helper.cc

Index: ash/mus/test/wm_test_helper.cc
diff --git a/ash/mus/test/wm_test_helper.cc b/ash/mus/test/wm_test_helper.cc
index 12a79528603f9824ba73be8b944b7b3b0fb17a6a..9f0456e3af20061cb1be82dbe651abb6eb4e02d5 100644
--- a/ash/mus/test/wm_test_helper.cc
+++ b/ash/mus/test/wm_test_helper.cc
@@ -6,6 +6,7 @@
 
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/test/material_design_controller_test_api.h"
+#include "ash/common/test/test_new_window_client.h"
 #include "ash/common/test/test_system_tray_delegate.h"
 #include "ash/common/test/wm_shell_test_api.h"
 #include "ash/common/wm_shell.h"
@@ -110,6 +111,7 @@ void WmTestHelper::Init() {
   // create the various test delegates.
   WmShellTestApi().SetSystemTrayDelegate(
       base::MakeUnique<test::TestSystemTrayDelegate>());
+  WmShellTestApi().SetNewWindowClient(base::MakeUnique<TestNewWindowClient>());
 
   ui::WindowTreeClient* window_tree_client =
       window_manager_app_->window_manager()->window_tree_client();