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

ui/app_list/app_list_folder_item.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/app_list/app_list_folder_item.h"
 
 #include "base/guid.h"
 #include "ui/app_list/app_list_constants.h"
 #include "ui/app_list/app_list_item_list.h"
 #include "ui/gfx/canvas.h"
@@ skipping 18 matching lines @@
     DCHECK(icons.size() <= kNumFolderTopItems);
   }
 
   virtual ~FolderImageSource() {}
 
  private:
   void DrawIcon(gfx::Canvas* canvas,
                 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));
     canvas->DrawImageInt(resized, 0, 0, resized.width(), resized.height(),
         x, y, resized.width(), resized.height(), true);
   }
 
   // gfx::CanvasImageSource overrides:
   virtual void Draw(gfx::Canvas* canvas) OVERRIDE {
     // Draw folder circle.
@@ skipping 198 matching lines @@
   for (size_t i = 0;
        i < kNumFolderTopItems && i < item_list_->item_count(); ++i) {
     AppListItem* item = item_list_->item_at(i);
     item->AddObserver(this);
     top_items_.push_back(item);
   }
   UpdateIcon();
 }
 
 }  // namespace app_list