Ash Tray: RemovePrivacyScreenToastController Obs in Dtor

This CL fixes a bug in which we do not remove
PrivacyScreenToastController as an observer of PrivacyScreenController
during destruction.

Bug: b:157775487, 1090385
Change-Id: I97f0740a2a97493750e2f751ae2eb390b743df2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225863Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774314}

ash/system/privacy_screen/privacy_screen_toast_controller.cc

@@ -26,6 +26,7 @@ PrivacyScreenToastController::~PrivacyScreenToastController() {
   close_timer_.Stop();
   if (bubble_widget_ && !bubble_widget_->IsClosed())
     bubble_widget_->CloseNow();
+  Shell::Get()->privacy_screen_controller()->RemoveObserver(this);
 }
 
 void PrivacyScreenToastController::ShowToast() {