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

mash/common/config.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 "mash/common/config.h"
+
+#include "base/command_line.h"
+
+namespace mash {
+namespace common {
+
+const char kWindowManagerSwitch[] = "window-manager";
+
+std::string GetWindowManagerServiceName() {
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(kWindowManagerSwitch)) {
+    std::string service_name =
+        base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
+            kWindowManagerSwitch);
+    return service_name;
+  }
+  // TODO(beng): move this constant to a mojom file in //ash.
+  return "ash";
+}
+
+}  // namespace common
+}  // namespace mash