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The signin-allowed policy is now tied to the new
pref signin.allowed_on_next_startup.
This means that changes only take effect on the next
startup.

Note: This change only affects Desktop.

Additional change:
 - Move the signin-allowed toggle to the top of the
   privacy page.

Screenshot:
https://drive.google.com/file/d/1FD0gQRmwlnIFeiAMpU7U_3x4ErUu91Tb/view?usp=sharing

TBR=scottchen@chromium.org

Bug: 888776
Change-Id: I4f7281a3b7eee8e799c48cf829dde12c7e9cbcc8
Reviewed-on: https://chromium-review.googlesource.com/1249021Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594705}