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The SigninPromoViewMediator should be in charge of registering and using:
  - prefs::kIosBookmarkSigninPromoDisplayedCount
  - prefs::kIosBookmarkPromoAlreadySeen

Also, once the sign-in promo view has been seen too much by the user, it should not set
prefs::kIosBookmarkPromoAlreadySeen to true. kIosBookmarkPromoAlreadySeen should be used only for
the close button. Some views could have a max number of views, without a close button.

This work is to prepare the clean up for the sign-in promo view in the settings
(crrev.com/c/774519).

Bug: 772050
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Id7f7cfc1146afbec552aaa159b628caac691d105
Reviewed-on: https://chromium-review.googlesource.com/776780Reviewed-by: Eric Noyau <noyau@chromium.org>
Reviewed-by: Louis Romero <lpromero@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517845}