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

components/exo/wm_helper_mus.h

Index: components/exo/wm_helper_mus.h
diff --git a/components/exo/wm_helper_mus.h b/components/exo/wm_helper_mus.h
index 379c5d3f489e0a7798adfb6bdf2a2d99ed9ca761..6a6f6fe59ab89eea1cefad9b88aabd1bb28980ba 100644
--- a/components/exo/wm_helper_mus.h
+++ b/components/exo/wm_helper_mus.h
@@ -9,15 +9,11 @@
 #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:
@@ -37,10 +33,6 @@ class WMHelperMus : public WMHelper,
   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;
@@ -50,14 +42,6 @@ class WMHelperMus : public WMHelper,
                        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_;