Clear item ids in HoldingSpaceTrayIcon::Clear

Also, clear the tray icon when hiding previews (as this cancels any
pending update timers).

BUG=None

Change-Id: I9647ff70b3359bd803a434d39b64d3cd6f1186b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631193Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843875}

ash/system/holding_space/holding_space_tray.cc

@@ -394,8 +394,10 @@ void HoldingSpaceTray::UpdatePreviewsVisibility() {
   DCHECK(previews_tray_icon_);
   previews_tray_icon_->SetVisible(show_previews);
 
-  if (!show_previews)
+  if (!show_previews) {
+    previews_tray_icon_->Clear();
     previews_update_.Stop();
+  }
 }
 
 void HoldingSpaceTray::SchedulePreviewsIconUpdate() {

ash/system/holding_space/holding_space_tray_icon.cc

@@ -129,6 +129,7 @@ HoldingSpaceTrayIcon::~HoldingSpaceTrayIcon() = default;
 
 void HoldingSpaceTrayIcon::Clear() {
   previews_update_weak_factory_.InvalidateWeakPtrs();
+  item_ids_.clear();
   previews_by_id_.clear();
   removed_previews_.clear();
   SetPreferredSize(CalculatePreferredSize());