Refactor backdrop that is currently used in the maximized mode.

components/exo/shell_surface.h

Index: components/exo/shell_surface.h
diff --git a/components/exo/shell_surface.h b/components/exo/shell_surface.h
index 7443e48dc2d7696b23b2286d1a0335ac14381626..132508ed4802cba49a756b7b3d89d1d3803c770c 100644
--- a/components/exo/shell_surface.h
+++ b/components/exo/shell_surface.h
@@ -49,7 +49,6 @@ class ShellSurface : public SurfaceDelegate,
                      public ash::wm::WindowStateObserver,
                      public aura::WindowObserver,
                      public WMHelper::ActivationObserver,
-                     public WMHelper::AccessibilityObserver,
                      public WMHelper::DisplayConfigurationObserver {
  public:
   enum class BoundsMode { SHELL, CLIENT, FIXED };
@@ -263,9 +262,6 @@ class ShellSurface : public SurfaceDelegate,
       aura::Window* gained_active,
       aura::Window* lost_active) override;
 
-  // Overridden from WMHelper::AccessibilityObserver:
-  void OnAccessibilityModeChanged() override;
-
   // Overridden from WMHelper::DisplayConfigurationObserver:
   void OnDisplayConfigurationChanged() override;
 
@@ -367,7 +363,6 @@ class ShellSurface : public SurfaceDelegate,
   int pending_resize_component_ = HTCAPTION;
   std::unique_ptr<aura::Window> shadow_overlay_;
   std::unique_ptr<aura::Window> shadow_underlay_;
-  std::unique_ptr<ui::EventHandler> shadow_underlay_event_handler_;
   gfx::Rect shadow_content_bounds_;
   float shadow_background_opacity_ = 1.0;
   std::deque<Config> pending_configs_;