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This CL adds SinginPromoMediator which is used to configure the signin
promo. This class is then used in SigninPreference, making the promo
visibile in the Settings page.

Bug: 737743
Change-Id: I7f5587b32a377db603cf536f8cb1cd9ba8851fdb
Reviewed-on: https://chromium-review.googlesource.com/576169Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Iulia Harasim <iuliah@google.com>
Cr-Commit-Position: refs/heads/master@{#488673}