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

ash/wm/panels/panel_frame_view.h

Index: ash/wm/panels/panel_frame_view.h
diff --git a/ash/wm/panels/panel_frame_view.h b/ash/wm/panels/panel_frame_view.h
index 0ecd7e7db6c9596a8c1c0b84fc41006350d6d636..1c0547a2f51a56bc830a42d0bb021fc8b3fe0f05 100644
--- a/ash/wm/panels/panel_frame_view.h
+++ b/ash/wm/panels/panel_frame_view.h
@@ -8,6 +8,7 @@
 #include <memory>
 
 #include "ash/ash_export.h"
+#include "ash/common/shell_observer.h"
 #include "base/macros.h"
 #include "ui/views/window/non_client_view.h"
 
@@ -20,7 +21,8 @@ class DefaultHeaderPainter;
 class FrameCaptionButtonContainerView;
 class FrameBorderHitTestController;
 
-class ASH_EXPORT PanelFrameView : public views::NonClientFrameView {
+class ASH_EXPORT PanelFrameView : public views::NonClientFrameView,
+                                  public ShellObserver {
  public:
   // Internal class name.
   static const char kViewClassName[];
@@ -34,7 +36,7 @@ class ASH_EXPORT PanelFrameView : public views::NonClientFrameView {
   // will have some transparency added when the frame is drawn.
   void SetFrameColors(SkColor active_frame_color, SkColor inactive_frame_color);
 
-  // Overridden from views::View:
+  // views::View:
   const char* GetClassName() const override;
 
  private:
@@ -43,7 +45,7 @@ class ASH_EXPORT PanelFrameView : public views::NonClientFrameView {
   // Height from top of window to top of client area.
   int NonClientTopBorderHeight() const;
 
-  // Overridden from views::NonClientFrameView:
+  // views::NonClientFrameView:
   gfx::Rect GetBoundsForClientView() const override;
   gfx::Rect GetWindowBoundsForClientBounds(
       const gfx::Rect& client_bounds) const override;
@@ -54,11 +56,15 @@ class ASH_EXPORT PanelFrameView : public views::NonClientFrameView {
   void UpdateWindowTitle() override;
   void SizeConstraintsChanged() override;
 
-  // Overridden from views::View:
+  // views::View:
   gfx::Size GetMinimumSize() const override;
   void Layout() override;
   void OnPaint(gfx::Canvas* canvas) override;
 
+  // ShellObserver:
+  void OnOverviewModeStarting() override;
+  void OnOverviewModeEnded() override;
+
   // Child View class describing the panel's title bar behavior
   // and buttons, owned by the view hierarchy
   views::Widget* frame_;