[ash-md] Improves smoothness with many windows in overview

ash/common/wm/overview/window_selector_item.h

 // Copyright 2013 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_COMMON_WM_OVERVIEW_WINDOW_SELECTOR_ITEM_H_
 #define ASH_COMMON_WM_OVERVIEW_WINDOW_SELECTOR_ITEM_H_
 
 #include <memory>
 
 #include "ash/ash_export.h"
@@ skipping 96 matching lines @@
 
   // Closes |transform_window_|.
   void CloseWindow();
 
   // Sets if the item is dimmed in the overview. Changing the value will also
   // change the visibility of the transform windows.
   void SetDimmed(bool dimmed);
   bool dimmed() const { return dimmed_; }
 
   const gfx::Rect& target_bounds() const { return target_bounds_; }
+  static void set_use_mask(bool use_mask) { use_mask_ = use_mask; }
+  static void set_use_shape(bool use_shape) { use_shape_ = use_shape; }
 
   // views::ButtonListener:
   void ButtonPressed(views::Button* sender, const ui::Event& event) override;
 
   // WmWindowObserver:
   void OnWindowDestroying(WmWindow* window) override;
   void OnWindowTitleChanged(WmWindow* window) override;
 
  private:
   class CaptionContainerView;
@@ skipping 20 matching lines @@
   // |animation_type|.
   void UpdateHeaderLayout(OverviewAnimationType animation_type);
 
   // Animates opacity of the |transform_window_| and its caption to |opacity|
   // using |animation_type|.
   void AnimateOpacity(float opacity, OverviewAnimationType animation_type);
 
   // Updates the close buttons accessibility name.
   void UpdateCloseButtonAccessibilityName();
 
+  static bool hide_header() { return use_mask_ || use_shape_; }
+
   // True if the item is being shown in the overview, false if it's being
   // filtered.
   bool dimmed_;
 
   // The root window this item is being displayed on.
   WmWindow* root_window_;
 
   // The contained Window's wrapper.
   ScopedTransformOverviewWindow transform_window_;
 
@@ skipping 29 matching lines @@
 
   // A close button for the window in this item. Owned by the
   // |caption_container_view_| with Material Design or by |close_button_widget_|
   // otherwise.
   views::ImageButton* close_button_;
 
   // Pointer to the WindowSelector that owns the WindowGrid containing |this|.
   // Guaranteed to be non-null for the lifetime of |this|.
   WindowSelector* window_selector_;
 
+  // If true, mask the original window header while in overview and make corners
+  // rounded using a mask layer. This has performance implications so it can be
+  // disabled when there are many windows.
+  static bool use_mask_;
+
+  // If true, hide the original window header while in overview using alpha
+  // shape. This has performance implications so it can be disabled when there
+  // are many windows.
+  static bool use_shape_;
+
   DISALLOW_COPY_AND_ASSIGN(WindowSelectorItem);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_OVERVIEW_WINDOW_SELECTOR_ITEM_H_