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

ash/common/wm/overview/window_selector_item.h

Index: ash/common/wm/overview/window_selector_item.h
diff --git a/ash/common/wm/overview/window_selector_item.h b/ash/common/wm/overview/window_selector_item.h
index bf62659ca3f491090ec49e84d788cf4f0dc88139..e95941967268f043a3b62f944f24074f9ecdb298 100644
--- a/ash/common/wm/overview/window_selector_item.h
+++ b/ash/common/wm/overview/window_selector_item.h
@@ -112,6 +112,7 @@ class ASH_EXPORT WindowSelectorItem : public views::ButtonListener,
   // change the visibility of the transform windows.
   void SetDimmed(bool dimmed);
   bool dimmed() const { return dimmed_; }
+  void set_hide_header(bool hide_header) { hide_header_ = hide_header; }
 
   const gfx::Rect& target_bounds() const { return target_bounds_; }
 
@@ -172,6 +173,10 @@ class ASH_EXPORT WindowSelectorItem : public views::ButtonListener,
   // a window layer for display on another monitor.
   bool in_bounds_update_;
 
+  // If true, mask the original window header while in overview. This has
+  // performance implications so it can be disabled when there are many windows.
+  bool hide_header_;
+
   // Label displaying its name (active tab for tabbed windows).
   // With Material Design this Widget owns |caption_container_view_| and is
   // shown above the |transform_window_|.