Add dummy implementation of message box for Windows.

browser/message_box_win.cc

@@ -0,0 +1,18 @@
+// Copyright (c) 2013 GitHub, Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "browser/message_box.h"
+
+namespace atom {
+
+int ShowMessageBox(NativeWindow* parent_window,
+                   MessageBoxType type,
+                   const std::vector<std::string>& buttons,
+                   const std::string& title,
+                   const std::string& message,
+                   const std::string& detail) {
+  return 0;
+}
+
+}  // namespace atom