Refactors keyboard related code so mash can use a keyboard

ash/mojo_shell.h

+// 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.
+
+#ifndef ASH_MOJO_SHELL_H_
+#define ASH_MOJO_SHELL_H_
+
+#include "ash/ash_export.h"
+
+namespace mojo {
+class Shell;
+}
+
+// NOTE: this code is always compiled (gn and gyp) and the gyp variant always
+// returns null for the shell.
+namespace ash {
+
+// TODO(sky): better isolate this. It should only be called during startup.
+ASH_EXPORT void SetMojoShell(mojo::Shell* shell);
+ASH_EXPORT mojo::Shell* GetMojoShell();
+
+}  // namespace ash
+
+#endif  // ASH_MOJO_SHELL_H_