Shell: Make a separate binary for child processes.

shell/child_main.h

-// Copyright 2014 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 SHELL_CHILD_MAIN_H_
-#define SHELL_CHILD_MAIN_H_
-
-namespace mojo {
-namespace shell {
-
-// The "main()" for child processes. This should be called with a
-// the |base::CommandLine| singleton initialized and a |base::AtExitManager|,
-// but without a |base::MessageLoop| for the current thread. Returns the exit
-// code for the process.
-int ChildMain();
-
-}  // namespace shell
-}  // namespace mojo
-
-#endif  // SHELL_CHILD_MAIN_H_