Replaced BackgroundAnimator with ShelfBackgroundAnimator.

ash/system/tray/tray_background_view.cc

Index: ash/system/tray/tray_background_view.cc
diff --git a/ash/system/tray/tray_background_view.cc b/ash/system/tray/tray_background_view.cc
index dfa9742dff48a9b450a6ac2ef9ee3e0595bac9b4..721ae1417711bf0a846a14b508d78be5bd299c82 100644
--- a/ash/system/tray/tray_background_view.cc
+++ b/ash/system/tray/tray_background_view.cc
@@ -378,7 +378,8 @@ void TrayBackgroundView::OnGestureEvent(ui::GestureEvent* event) {
   ActionableView::OnGestureEvent(event);
 }
 
-void TrayBackgroundView::UpdateBackground(int alpha) {
+void TrayBackgroundView::UpdateBackground(BackgroundAnimator* animator,
+                                          int alpha) {
   // The animator should never fire when the alternate shelf layout is used.
   if (!background_ || draw_background_as_active_)
     return;