Don't reposition if shutdown is in progress

ash/common/wm/overview/window_grid.cc

Index: ash/common/wm/overview/window_grid.cc
diff --git a/ash/common/wm/overview/window_grid.cc b/ash/common/wm/overview/window_grid.cc
index f1a5b4637f07ab05b0b47e919aff74988b1d9084..bb13d0f5c77891943458ec3d77ec147712adf27c 100644
--- a/ash/common/wm/overview/window_grid.cc
+++ b/ash/common/wm/overview/window_grid.cc
@@ -607,6 +607,9 @@ void WindowGrid::PositionWindowsMD(bool animate) {
 }
 
 void WindowGrid::PositionWindows(bool animate) {
+  if (window_selector_->is_shut_down())
+    return;
+
   if (ash::MaterialDesignController::IsOverviewMaterial()) {
     DCHECK(shield_widget_.get());
     // Keep the background shield widget covering the whole screen.