mash: Port TrayUpdateTest to AshTest

ash/common/test/wm_shell_test_api.cc

+// Copyright 2016 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.
+
+#include "ash/common/test/wm_shell_test_api.h"
+
+#include "ash/common/system/tray/system_tray_delegate.h"
+#include "ash/common/wm_shell.h"
+
+namespace ash {
+
+WmShellTestApi::WmShellTestApi() {}
+
+WmShellTestApi::~WmShellTestApi() {}
+
+void WmShellTestApi::SetSystemTrayDelegate(
+    std::unique_ptr<SystemTrayDelegate> delegate) {
+  WmShell::Get()->SetSystemTrayDelegate(std::move(delegate));
+}
+
+}  // namespace ash