Fix page slip issue after dragging

Changes:
Update the selected page to be the target page of dragged item at the
end of dragging.

Bug: 893832
Change-Id: I91dcf8e03f62eab1597e3899bdbcafdad1fec2df
Reviewed-on: https://chromium-review.googlesource.com/c/1324851
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606585}

ash/app_list/views/apps_grid_view.cc

@@ -738,6 +738,13 @@ void AppsGridView::EndDrag(bool cancel) {
                                  drag_source_bounds);
   }
 
+  if (!cancel) {
+    // Select the page where dragged item is dropped.
+    pagination_model_.SelectPage(
+        GetIndexFromModelIndex(GetModelIndexOfItem(drag_item)).page,
+        false /* animate */);
+  }
+
   StopPageFlipTimer();
 }
 
@@ -2199,9 +2206,6 @@ void AppsGridView::MoveItemInModel(AppListItemView* item_view,
   view_model_.Move(current_model_index, target_model_index);
   item_list_->AddObserver(this);
 
-  if (pagination_model_.selected_page() != target.page)
-    pagination_model_.SelectPage(target.page, false);
-
   RecordAppMovingTypeMetrics(folder_delegate_ ? kReorderInFolder
                                               : kReorderInTopLevel);
 }