CrOS Shelf: Guard against a potential null dereference in |ShelfWidget|

This is a tentative fix for the linked bug.

Bug: 1010699
Change-Id: I95e24169ada31291d9a1be1598c0887d39f11e46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836597
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Auto-Submit: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702254}

ash/shelf/shelf_widget.cc

@@ -215,6 +215,7 @@ void ShelfWidget::DelegateView::UpdateOpaqueBackground() {
   const ShelfBackgroundType background_type =
       shelf_widget_->GetBackgroundType();
   const bool tablet_mode =
+      Shell::Get()->tablet_mode_controller() &&
       Shell::Get()->tablet_mode_controller()->InTabletMode();
   const bool in_app = ShelfConfig::Get()->is_in_app();