aura: Remove client::UserActionClient.

ui/aura/client/user_action_client.h

-// Copyright (c) 2012 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 UI_AURA_CLIENT_USER_ACTION_CLIENT_H_
-#define UI_AURA_CLIENT_USER_ACTION_CLIENT_H_
-
-#include "ui/aura/aura_export.h"
-
-namespace aura {
-class Window;
-namespace client {
-
-// An interface for handling a user action that isn't handled by the standard
-// event path.
-class AURA_EXPORT UserActionClient {
- public:
-  enum Command {
-    BACK = 0,
-    FORWARD,
-  };
-
-  // Returns true if the command was handled and false otherwise.
-  virtual bool OnUserAction(Command command) = 0;
-
-  virtual ~UserActionClient() {}
-};
-
-// Sets/gets the client for handling user action on the specified root window.
-AURA_EXPORT void SetUserActionClient(Window* root_window,
-                                     UserActionClient* client);
-AURA_EXPORT UserActionClient* GetUserActionClient(Window* root_window);
-
-}  // namespace client
-}  // namespace aura
-
-#endif  // UI_AURA_CLIENT_USER_ACTION_CLIENT_H_