Replace menu_bg and edge_menu_bg with a new modern popup_bg.

BUG=823531

Change-Id: Idec73e189527363cb8186e006e22657a4b5dad8d
Reviewed-on: https://chromium-review.googlesource.com/1108924Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Becky Zhou <huayinz@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569660}