exo: Use aura::Env()->Add{Pre,Post}TargetHandler to simplfy WmHelperMus.

components/exo/wm_helper_mus.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 COMPONENTS_EXO_WM_HELPER_MUS_H_
 #define COMPONENTS_EXO_WM_HELPER_MUS_H_
 
 #include "base/macros.h"
 #include "components/exo/wm_helper.h"
 #include "services/ui/public/cpp/window_tree_client_observer.h"
 #include "ui/aura/client/focus_change_observer.h"
-#include "ui/aura/env_observer.h"
-#include "ui/events/event_handler.h"
 
 namespace exo {
 
 // A helper class for accessing WindowManager related features.
 class WMHelperMus : public WMHelper,
-                    public aura::EnvObserver,
-                    public ui::EventHandler,
                     public ui::WindowTreeClientObserver,
                     public aura::client::FocusChangeObserver {
  public:
   WMHelperMus();
   ~WMHelperMus() override;
 
   // Overriden from WMHelper:
   const ash::DisplayInfo GetDisplayInfo(int64_t display_id) const override;
   aura::Window* GetContainer(int container_id) override;
   aura::Window* GetActiveWindow() const override;
   aura::Window* GetFocusedWindow() const override;
   ui::CursorSetType GetCursorSet() const override;
   void AddPreTargetHandler(ui::EventHandler* handler) override;
   void PrependPreTargetHandler(ui::EventHandler* handler) override;
   void RemovePreTargetHandler(ui::EventHandler* handler) override;
   void AddPostTargetHandler(ui::EventHandler* handler) override;
   void RemovePostTargetHandler(ui::EventHandler* handler) override;
   bool IsMaximizeModeWindowManagerEnabled() const override;
 
-  // Overriden from aura::EnvObserver:
-  void OnWindowInitialized(aura::Window* window) override {}
-  void OnHostInitialized(aura::WindowTreeHost* host) override;
-
   // Overriden from ui::WindowTreeClientObserver:
   void OnWindowTreeFocusChanged(ui::Window* gained_focus,
                                 ui::Window* lost_focus) override;
 
   // Overriden from ui::client::FocusChangeObserver:
   void OnWindowFocused(aura::Window* gained_focus,
                        aura::Window* lost_focus) override;
 
  private:
-  class EventForwarder;
-
-  ui::EventHandlerList pre_target_list_;
-  ui::EventHandlerList post_target_list_;
-
-  std::unique_ptr<EventForwarder> pre_target_event_forwarder_;
-  std::unique_ptr<EventForwarder> post_target_event_forwarder_;
-
   aura::Window* active_window_;
   aura::Window* focused_window_;
 
   DISALLOW_COPY_AND_ASSIGN(WMHelperMus);
 };
 
 }  // namespace exo
 
 #endif  // COMPONENTS_EXO_WM_HELPER_H_