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This reverts commit 468ce86a.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 587137 as the
culprit for failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtLzQ2OGNlODZhMDEwY2U4MzEzZTZiOTJhNGZmYjhhNGEzODYxODkzNDIM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.memory/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%281%29/28784

Sample Failed Step: ash_unittests

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}

Change-Id: I92133a3f12ad29bd54f2d20f8f1bba53b7d1762a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 878601
Reviewed-on: https://chromium-review.googlesource.com/1196027
Cr-Commit-Position: refs/heads/master@{#587354}