applist: Fix issue with blackbox showing behind folders animating.

The blackbox shows up when ending a drag which creates a folder.
The folder view should be painted to a layer with set bounds
opaquely=false, so that the following animation (which will create
a layer with set bounds opaquely=true) will not create a blackbox.

Test: manual
Bug: 1057955
Change-Id: Ida8ea89e131e64dc2ed8d34465e82ba6a5745318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086261Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747858}

ash/app_list/views/apps_grid_view.cc

@@ -726,6 +726,10 @@ void AppsGridView::EndDrag(bool cancel) {
           DropTargetIsValidFolder()) {
         MaybeCreateFolderDroppingAccessibilityEvent();
         folder_item_view = MoveItemToFolder(drag_view_, drop_target_);
+        // If the view that the folder is replacing had a layer, ensure the new
+        // folder view has one too.
+        if (drag_view_ && drag_view_->layer())
+          folder_item_view->EnsureLayer();
       } else if (IsValidReorderTargetIndex(drop_target_)) {
         // Ensure reorder event has already been announced by the end of drag.
         MaybeCreateDragReorderAccessibilityEvent();