Add dummy implementations for TrayIcon.

atom/browser/ui/tray_icon.cc

@@ -0,0 +1,19 @@
+// Copyright (c) 2014 GitHub, Inc. All rights reserved.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#include "atom/browser/ui/tray_icon.h"
+
+namespace atom {
+
+TrayIcon::TrayIcon() : model_(NULL) {
+}
+
+TrayIcon::~TrayIcon() {
+}
+
+void TrayIcon::SetContextMenu(ui::SimpleMenuModel* menu_model) {
+  model_ = menu_model;
+}
+
+}  // namespace atom