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1) The new wallpaper picker relies on the Backdrop server to select
   surprise me images, instead of selecting random images by ourseleves
   (the back-end was implemented by a prior CL).

2) The surprise me images are per-category, ie. each wallpaper category
   has a "daily refresh slider". At most one slider may be active at
   a time. Toggling one slider to active should deactivate others.

3) Use "daily refresh" instead of "surprise me" in the code to
   distinguish from the old picker. Although it's possible to share
   code path with the old picker, generally avoid it for clarity.

Test steps include:
1) Toggle daily refresh between enabled/disabled, during
   tablet/clamshell mode, and among different categories.
2) Enable surprise me on the old picker, then add switch
   --new-wallpaper-picker, and see the migration.
3) See the sync is working for two devices with the same account id.

Bug: 834998, 810169, 834994, 837338
Change-Id: I369ce57ac4bd07e91da31bb2971b6970501f9c3a
Reviewed-on: https://chromium-review.googlesource.com/1093742
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568732}