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This adds a class, ReopenTabPromoController, that manages display of
the reopen tab in-product help promo, and expands the interfaces of
AppMenu and AppMenuButton to allow this.

Change-Id: I78431981a21d432778aceb65d3aa17642164bc14
Bug: 887991
Reviewed-on: https://chromium-review.googlesource.com/c/1334937
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609405}