Ash Tray: Fix crash after pressing ESC with message center focused.

This regression was caused by refactoring the delegate in
https://chromium-review.googlesource.com/c/chromium/src/+/1938253

Change-Id: I2fe052bbe240b9fb19ef0c6c5bd633f8d67f2336
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1940870Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721704}

ash/system/message_center/unified_message_center_bubble.cc

@@ -115,6 +115,7 @@ UnifiedMessageCenterBubble::~UnifiedMessageCenterBubble() {
     CHECK(message_center_view_);
     message_center_view_->RemoveObserver(this);
 
+    bubble_view_->ResetDelegate();
     bubble_widget_->RemoveObserver(this);
     bubble_widget_->CloseNow();
   }
@@ -208,6 +209,7 @@ void UnifiedMessageCenterBubble::OnWidgetDestroying(views::Widget* widget) {
   message_center_view_->RemoveObserver(this);
   bubble_widget_->RemoveObserver(this);
   bubble_widget_ = nullptr;
+  bubble_view_->ResetDelegate();
 
   // Close the quick settings bubble as well, which may not automatically happen
   // when dismissing the message center bubble by pressing ESC.