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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}