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This is a reland of 139a79ec
Original change's description:
> [Suggestions] Remove CTAs when feature is disabled
>
> Stops showing the Sign in Promo and the All Dismissed item when remote
> suggestions are disabled and performing the advertised actions would not
> provide the user with new suggestions.
>
> Part of this CL involves making the SuggestionsSources notifies a list
> of observer rather than a single one.
>
> Bug: 738872
> Change-Id: I16c300fb14a1d5780578387d48d848570339befe
> Reviewed-on: https://chromium-review.googlesource.com/567182
> Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
> Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#486034}

Bug: 738872
Change-Id: Ifc5f6643b4402657b9a62e08596d9e3267ce79a8
Reviewed-on: https://chromium-review.googlesource.com/571722Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486723}