Ash Notification: Add SetPaintToLayer to stacked notification bar.

The regression bug below was caused by ff070b8e where we changed
SetPaintToLayer in TrayBubbleView. Added this to stacked notification
bar to fix the issue.

BUG=1116396

Change-Id: Ied59608d5cce78396a8352faf9d583ca877de74a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2372103Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: Andre Le <leandre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801419}

ash/system/message_center/stacked_notification_bar.cc

@@ -284,6 +284,8 @@ StackedNotificationBar::StackedNotificationBar(
 
   expand_all_button_->SetVisible(false);
   AddChildView(expand_all_button_);
+
+  SetPaintToLayer();
 }
 
 StackedNotificationBar::~StackedNotificationBar() {