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This implements the user image picker to look and behave the same
as the Settings UI.

Notes:
* Because we use the same icon and camera size, the icons overflow
  and scroll. This is consistent with the new UX so seems acceptable
  as an intermediate step.
* The supervised user picker has not been changed yet, pending the
  status of the feature.

Bug: 730031
Change-Id: Iaa1044e5eb57f37677d52ba592a7fcb470803abe
Reviewed-on: https://chromium-review.googlesource.com/569311
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486959}