CrOS Shelf: Status area isn't actually observing shelf bg

The |StatusAreaWidget| class implements this observer interface and
gets registered as an observer but it never does anything with it.

Bug: 973443
Change-Id: I060cc3477587edfec2d9ecd9f79496a517d9c774
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679432Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672695}

ash/shelf/shelf_widget.cc

@@ -398,7 +398,6 @@ void ShelfWidget::Shutdown() {
   // access it later in shutdown.
   shelf_layout_manager_->PrepareForShutdown();
 
-  background_animator_.RemoveObserver(status_area_widget_.get());
   Shell::Get()->focus_cycler()->RemoveWidget(status_area_widget_.get());
   status_area_widget_.reset();
 
@@ -421,7 +420,6 @@ void ShelfWidget::CreateStatusAreaWidget(aura::Window* status_container) {
       std::make_unique<StatusAreaWidget>(status_container, shelf_);
   status_area_widget_->Initialize();
   Shell::Get()->focus_cycler()->AddWidget(status_area_widget_.get());
-  background_animator_.AddObserver(status_area_widget_.get());
   status_container->SetLayoutManager(new StatusAreaLayoutManager(this));
 }
 

ash/system/status_area_widget.h

@@ -9,7 +9,6 @@
 #include "ash/kiosk_next/kiosk_next_shell_observer.h"
 #include "ash/login_status.h"
 #include "ash/public/cpp/shelf_types.h"
-#include "ash/shelf/shelf_background_animator_observer.h"
 #include "base/macros.h"
 #include "ui/views/widget/widget.h"
 
@@ -35,7 +34,6 @@ class VirtualKeyboardTray;
 // so that it can be shown in cases where the rest of the shelf is hidden (e.g.
 // on secondary monitors at the login screen).
 class ASH_EXPORT StatusAreaWidget : public views::Widget,
-                                    public ShelfBackgroundAnimatorObserver,
                                     public KioskNextShellObserver {
  public:
   StatusAreaWidget(aura::Window* status_container, Shelf* shelf);