Update animating shelf background's drag handle bounds

Updates how animating shelf background's drag handle layer bounds are
calculated. Instead of directly using bounds from that the drag handle
view has within the opaque shelf widget, calculate the animating drag
handle bounds directly by centering the drag handle within the
animating background bounds.

Opaque background bounds are calculated based on the shelf widget size,
which may differ from the animating (in-app) shelf size (e.g. on home
screen). Using the opaque background drag handle bounds for animating
background would displace the drag handle when that is the case.

Change-Id: I51cb9a4d031286329784162193627bb09c608043
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079839Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745497}

ash/shelf/hotseat_transition_animator.cc

@@ -118,8 +118,10 @@ void HotseatTransitionAnimator::DoAnimation(HotseatState old_state,
 
   shelf_widget_->GetAnimatingBackground()->SetColor(
       ShelfConfig::Get()->GetMaximizedShelfColor());
-  shelf_widget_->GetAnimatingDragHandle()->SetBounds(
-      shelf_widget_->GetDragHandle()->bounds());
+
+  gfx::Rect drag_handle_bounds(shelf_widget_->GetAnimatingBackground()->size());
+  drag_handle_bounds.ClampToCenteredSize(ShelfConfig::Get()->DragHandleSize());
+  shelf_widget_->GetAnimatingDragHandle()->SetBounds(drag_handle_bounds);
 
   animation_metrics_reporter_->set_new_state(new_state);