Remove duplication between Mandoline and mojo_runner main functions.

mojo/runner/desktop/main_helper.h

+// Copyright 2015 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 MOJO_RUNNER_DESKTOP_MAIN_HELPER_H
+#define MOJO_RUNNER_DESKTOP_MAIN_HELPER_H
+
+#include "base/callback.h"
+#include "url/gurl.h"
+
+namespace mojo {
+namespace runner {
+
+// Helper method to start Mojo Runner code.
+// If |mojo_url| is not empty, the given mojo application is started. Otherwise,
+// an application must have been specified on the command line and it is run.
+// |callback| is only called in the later case.
+int RunnerMain(int argc, char** argv,
+               const GURL& mojo_url,
+               const base::Closure& callback);
+
+}  // namespace runner
+}  // namespace mojo
+
+#endif  // MOJO_RUNNER_DESKTOP_MAIN_HELPER_H