Fix the issue that shelf is not usable when disabling scrollable shelf

It is a regression issue introduced by the CL on Hotseat. When the
flag is disabled, the init of DelegateView returns early then it fails
to set the layout manager.

Bug: 1009609
Change-Id: Id53ee51556e10d60985abcabbbcba186d5afcc01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838472Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702559}

ash/shelf/hotseat_widget.cc

@@ -85,12 +85,13 @@ HotseatWidget::DelegateView::~DelegateView() {
 void HotseatWidget::DelegateView::Init(
     ScrollableShelfView* scrollable_shelf_view,
     ui::Layer* parent_layer) {
+  SetLayoutManager(std::make_unique<views::FillLayout>());
+
   if (!chromeos::switches::ShouldShowScrollableShelf())
     return;
 
   if (wallpaper_controller_)
     wallpaper_controller_->AddObserver(this);
-  SetLayoutManager(std::make_unique<views::FillLayout>());
   SetParentLayer(parent_layer);
 
   DCHECK(scrollable_shelf_view);