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When ephemeral user selects a new wallpaper, it's not saved to disk.
But we should still be able to keep this wallpaper before user logout.
The wallpaper is already cached in memory, but we ignored the cache for
ephemeral users: so when lock screen or wallpaper app calls
ShowUserWallpaper, it always thinks that ephemeral users never set a
wallpaper and then display the default wallpaper.

Bug: 825237, 902327
Change-Id: Id9a811c91243b04c7459f3ee20569ec4fdff14b0
Reviewed-on: https://chromium-review.googlesource.com/c/1343105
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609844}