Refactors keyboard related code so mash can use a keyboard

ash/mojo_shell.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 "ash/mojo_shell.h"
+
+namespace ash {
+namespace {
+mojo::Shell* mojo_shell = nullptr;
+}  // namespace
+
+void SetMojoShell(mojo::Shell* shell) {
+  mojo_shell = shell;
+}
+
+mojo::Shell* GetMojoShell() {
+  return mojo_shell;
+}
+
+}  // namespace ash