cros: Check for layer existence before operating on it.

Fix for a crash that occurs while the AppList is closing. We assume the
layer exists, yet a few optimizations have been implemented which
may result in layers being destroyed when they are no longer needed.

Bug: 994409
Change-Id: I1d99f34c32e18ef70a3784b1e5337c1530cba872
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761308
Auto-Submit: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688312}

ash/app_list/views/apps_grid_view.cc

@@ -2729,6 +2729,8 @@ void AppsGridView::OnAppListModelStatusChanged() {
 void AppsGridView::SetViewHidden(AppListItemView* view,
                                  bool hide,
                                  bool immediate) {
+  if (!view->layer())
+    return;
   ui::ScopedLayerAnimationSettings animator(view->layer()->GetAnimator());
   animator.SetPreemptionStrategy(
       immediate ? ui::LayerAnimator::IMMEDIATELY_SET_NEW_TARGET