Creates BackgroundShell and uses it in test

mojo/shell/background/background_shell_main.cc

Index: mojo/shell/background/background_shell_main.cc
diff --git a/mojo/shell/background/background_shell_main.cc b/mojo/shell/background/background_shell_main.cc
new file mode 100644
index 0000000000000000000000000000000000000000..72c082f483a034d346190579077e3d70720cb848
--- /dev/null
+++ b/mojo/shell/background/background_shell_main.cc
@@ -0,0 +1,43 @@
+// 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 "mojo/shell/background/background_shell_main.h"
+
+#include "base/at_exit.h"
+#include "base/command_line.h"
+#include "base/debug/debugger.h"
+#include "base/process/launch.h"
+#include "mojo/shell/runner/common/switches.h"
+#include "mojo/shell/runner/host/child_process.h"
+#include "mojo/shell/runner/init.h"
+
+namespace mojo {
+namespace shell {
+namespace {
+
+int RunChildProcess() {
+  base::AtExitManager at_exit;
+  InitializeLogging();
+  WaitForDebuggerIfNecessary();
+#if !defined(OFFICIAL_BUILD) && defined(OS_WIN)
+  base::RouteStdioToConsole(false);
+#endif
+  return ChildProcessMain();
+}
+
+}  // namespace
+}  // shell
+}  // mojo
+
+int main(int argc, char** argv) {
+  base::CommandLine::Init(argc, argv);
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kChildProcess)) {
+    return mojo::shell::RunChildProcess();
+  }
+  // Reset CommandLine as most likely main() is going to use CommandLine too
+  // and expect to be able to initialize it.
+  base::CommandLine::Reset();
+  return MasterProcessMain(argc, argv);
+}