Fix stacked notification bar hide animation

The hide animation for stacked notification bar is not
correctly initiated in the current implementation.

Bug: 1051671
Change-Id: I94b75c161eddff9b6123e48c869e277a2e67ac31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067334Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743315}

ash/system/message_center/unified_message_center_view.cc

@@ -167,6 +167,12 @@ bool UnifiedMessageCenterView::IsNotificationBarVisible() {
 }
 
 void UnifiedMessageCenterView::OnNotificationSlidOut() {
+  if (collapsed_) {
+    if (!message_list_view_->GetTotalNotificationCount())
+      StartHideStackingBarAnimation();
+    return;
+  }
+
   if (notification_bar_->GetVisible() &&
       message_list_view_->GetTotalNotificationCount() <= 1) {
     StartHideStackingBarAnimation();