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This CL introduces the skeleton of a class which can drive the UI
of "blocked URL" lists (i.e. the popup and redirect setting bubbles).

Currently, it only exposes an observer interface to unify how popup
and redirect UI surfaces receive updates about new URLs that were
blocked.

Follow-up CLs can add functionality including storing the actual
blocked URLs within the manager.

Bug: 895555
Change-Id: I6e95cd4ca3c8d6109cbfdb770516dc6d18c98554
Reviewed-on: https://chromium-review.googlesource.com/c/1334420
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610035}