Add a unit test for resetting the app list on a reshow.

ui/app_list/app_list_folder_item.cc

Index: ui/app_list/app_list_folder_item.cc
diff --git a/ui/app_list/app_list_folder_item.cc b/ui/app_list/app_list_folder_item.cc
index 8a2080399ba19afabb15d415e3d8b2db99f69bb5..9554086bc82aa92b620123970764cc2ee7d8159e 100644
--- a/ui/app_list/app_list_folder_item.cc
+++ b/ui/app_list/app_list_folder_item.cc
@@ -36,6 +36,9 @@ class FolderImageSource : public gfx::CanvasImageSource {
                 const gfx::ImageSkia& icon,
                 const gfx::Size icon_size,
                 int x, int y) {
+    if (icon.isNull())
+      return;
+
     gfx::ImageSkia resized(
         gfx::ImageSkiaOperations::CreateResizedImage(
             icon, skia::ImageOperations::RESIZE_BEST, icon_size));