mash: Port TrayUpdateTest to AshTest

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 9a49ab61431d28783ab4563de48344eb30dc3c76..12a79528603f9824ba73be8b944b7b3b0fb17a6a 100644
--- a/ash/mus/test/wm_test_helper.cc
+++ b/ash/mus/test/wm_test_helper.cc
@@ -6,6 +6,9 @@
 
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/test/material_design_controller_test_api.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"
 #include "ash/mus/root_window_controller.h"
 #include "ash/mus/window_manager.h"
 #include "ash/mus/window_manager_application.h"
@@ -103,6 +106,11 @@ void WmTestHelper::Init() {
       window_tree_client_setup_.OwnWindowTreeClient(),
       blocking_pool_owner_->pool());
 
+  // TODO(jamescook): Pass a TestShellDelegate into WindowManager and use it to
+  // create the various test delegates.
+  WmShellTestApi().SetSystemTrayDelegate(
+      base::MakeUnique<test::TestSystemTrayDelegate>());
+
   ui::WindowTreeClient* window_tree_client =
       window_manager_app_->window_manager()->window_tree_client();
   window_tree_client_private_ =