[ash-md] Animates overview shield in and out

ash/common/wm/overview/window_grid.h

Index: ash/common/wm/overview/window_grid.h
diff --git a/ash/common/wm/overview/window_grid.h b/ash/common/wm/overview/window_grid.h
index f9678ff483ac6bc9d19e890830be2d19ab10fd19..cce8557c6d6c078f1beb9d27c095f5845cdd5f05 100644
--- a/ash/common/wm/overview/window_grid.h
+++ b/ash/common/wm/overview/window_grid.h
@@ -55,6 +55,9 @@ class ASH_EXPORT WindowGrid : public WmWindowObserver {
              WindowSelector* window_selector);
   ~WindowGrid() override;
 
+  // Exits overview mode, fading out the |shield_widget_| if necessary.
+  void Shutdown();
+
   // Prepares the windows in this grid for overview. This will restore all
   // minimized windows and ensure they are visible.
   void PrepareForOverview();