CrOS Shelf: Show back button opacity animation.

Without this change the back button just instantly appears/disappears.

Bug: 1031363
Change-Id: I731bfb62637215f18ea90c5c34b2b51d7be4c787
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1952423Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722585}

ash/shelf/shelf_navigation_widget.cc

@@ -336,6 +336,8 @@ void ShelfNavigationWidget::UpdateLayout() {
   ui::ScopedLayerAnimationSettings settings(
       GetBackButton()->layer()->GetAnimator());
   settings.SetTransitionDuration(animation_duration);
+  settings.SetPreemptionStrategy(
+      ui::LayerAnimator::IMMEDIATELY_ANIMATE_TO_NEW_TARGET);
   settings.AddObserver(this);
   GetBackButton()->layer()->SetOpacity(is_back_button_shown ? 1 : 0);