Make "Shut down" button clickable again on Welcome screen

Re-adding a line that initialized the layout manager. This line
was deleted in http://crrev.com/c/1817149 which led to the
"Shut down" button becoming not clickable.

Bug: 1014774
Test: manual
Change-Id: Ib2b8004a0c296f2d3355768e54bd9bc9b1331192
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891140Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711347}

ash/shelf/shelf_widget.cc

@@ -145,6 +145,7 @@ ShelfWidget::DelegateView::DelegateView(ShelfWidget* shelf_widget)
   DCHECK(shelf_widget_);
   set_owned_by_client();  // Deleted by DeleteDelegate().
 
+  SetLayoutManager(std::make_unique<views::FillLayout>());
   set_allow_deactivate_on_esc(true);
 
   std::unique_ptr<views::View> drag_handle_ptr =