Add the beginning of a simple window manager that we'll support on Windows.

mash/common/config.h

+// 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.
+
+#ifndef MASH_COMMON_CONFIG_H_
+#define MASH_COMMON_CONFIG_H_
+
+#include <string>
+
+namespace mash {
+namespace common {
+
+// This file contains configuration functions that can be used by any mash
+// service.
+
+// Returns the name of the window manager service to run. By default this will
+// return "ash". A different value can be specified on the command line by
+// passing --window-manager=<foo>.
+std::string GetWindowManagerServiceName();
+
+}  // namespace common
+}  // namespace mash
+
+#endif  // MASH_COMMON_CONFIG_H_