Reset holding space tray icon bounds when clearing.

Addresses the case where:
1. User has 3+ items in Tote
2. User batch removes all items via Files app
3. User adds back in item to Tote

Previously at step 3, the Tote tray icon bounds would jump to max size
on visibility change before animating back to min item bounds.

Change-Id: I8f15b584d0722b2601e69f63160befe83fc644d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2621974Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#842123}

ash/system/holding_space/holding_space_tray_icon.cc

@@ -119,6 +119,7 @@ void HoldingSpaceTrayIcon::Clear() {
   previews_update_weak_factory_.InvalidateWeakPtrs();
   previews_by_id_.clear();
   removed_previews_.clear();
+  SetPreferredSize(CalculatePreferredSize());
 }
 
 int HoldingSpaceTrayIcon::GetHeightForWidth(int width) const {