Don't attempt to cast holding space item view during destruction.

When `OnViewIsDeleting` is called, the holding space item view is
already well into its destruction sequence. We shouldn't attempt to cast
from `views::View` back to `HoldingSpaceItemView` at this time and we
have no need to.

Bug: 1137461
Change-Id: I3c431af03b692d15b4e83de25d9ffcaa2b35d148
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472584Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#817149}

ash/system/holding_space/holding_space_item_view_delegate.cc

@@ -243,7 +243,7 @@ void HoldingSpaceItemViewDelegate::WriteDragDataForView(
 }
 
 void HoldingSpaceItemViewDelegate::OnViewIsDeleting(views::View* view) {
-  base::Erase(views_, HoldingSpaceItemView::Cast(view));
+  base::Erase(views_, view);
   view_observer_.Remove(view);
 }