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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/567182Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486034}