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The original CL was reverted because of a test failure that was since
then fixed in CL 1194509

This is a reland of 468ce86a

Original change's description:
> CrOS Shelf: add background over app list
>
> Currently the shelf is fully transparent when the app
> list is open. In thr new UI, we want to make it slightly
> darker to distinguish it from the app list.
>
> Bug: 878601
> Change-Id: I5167e684e174c2d48334f45fa9b1c66e79ba32b7
> Reviewed-on: https://chromium-review.googlesource.com/1194067
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Commit-Queue: Manu Cornet <manucornet@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#587137}

TBR=xiyuan@chromium.org
No-Presubmit: true
No-Tree-Checks: true
No-Try: true

Bug: 878601
Change-Id: Ife5ed29f3c325de1665181cac2eb5fe1b39971fc
Reviewed-on: https://chromium-review.googlesource.com/1196144
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587358}