Reland "Fix page slip issue after dragging"

This is a reland of 05e6d611

Original change's description:
> 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}

Bug: 893832
Change-Id: I51c2d7d5d2a9a7ed4e752d71f3f6bee083ac84e0
Reviewed-on: https://chromium-review.googlesource.com/c/1329894Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606946}

ash/app_list/views/apps_grid_view.cc

@@ -738,6 +738,16 @@ void AppsGridView::EndDrag(bool cancel) {
                                  drag_source_bounds);
   }
 
+  if (!cancel) {
+    // Select the page where dragged item is dropped. Avoid doing so when the
+    // dragged item ends up in a folder.
+    const int model_index = GetModelIndexOfItem(drag_item);
+    if (model_index < view_model_.view_size()) {
+      pagination_model_.SelectPage(GetIndexFromModelIndex(model_index).page,
+                                   false /* animate */);
+    }
+  }
+
   StopPageFlipTimer();
 }
 
@@ -2201,9 +2211,6 @@ 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);
 }