AppListModel / AppsGridView: Better error handling for corner cases.

ui/app_list/views/apps_grid_view.cc

 // Copyright (c) 2012 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/views/apps_grid_view.h"
 
 #include <algorithm>
 #include <set>
 #include <string>
 
@@ skipping 1719 matching lines @@
 
   if (pagination_model_.selected_page() != target.page)
     pagination_model_.SelectPage(target.page, false);
 }
 
 void AppsGridView::MoveItemToFolder(AppListItemView* item_view,
                                     const Index& target) {
   const std::string& source_item_id = item_view->item()->id();
   AppListItemView* target_view =
       GetViewDisplayedAtSlotOnCurrentPage(target.slot);
+  DCHECK(target_view);
   const std::string& target_view_item_id = target_view->item()->id();
 
+  // Check that the item is not being dropped onto itself; this should not
+  // happen, but it can if something allows multiple views to share an
+  // item (e.g., if a folder drop does not clean up properly).
+  DCHECK_NE(source_item_id, target_view_item_id);
+
   // Make change to data model.
   item_list_->RemoveObserver(this);
   std::string folder_item_id =
       model_->MergeItems(target_view_item_id, source_item_id);
   item_list_->AddObserver(this);
   if (folder_item_id.empty()) {
     LOG(ERROR) << "Unable to merge into item id: " << target_view_item_id;
     return;
   }
   if (folder_item_id != target_view_item_id) {
@@ skipping 388 matching lines @@
 
 void AppsGridView::SetAsFolderDroppingTarget(const Index& target_index,
                                              bool is_target_folder) {
   AppListItemView* target_view =
       GetViewDisplayedAtSlotOnCurrentPage(target_index.slot);
   if (target_view)
     target_view->SetAsAttemptedFolderTarget(is_target_folder);
 }
 
 }  // namespace app_list