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This CL starts to refactor the PreviewsBlacklist.

The goal is to split Blacklist into it's own component and implement a
PreviewsBlackList as a derived class.

This CL creates core methods and Previews specific methods that can
easily be moved into seperate classes. Further, this cleans the
Blacklist  <-> OptOutStore API to pass a single object that supports
various blacklisting rules. This allows consumers to write their own
OptOutStore (or Blacklist) implementation, while getting all of the
functionality of the Blacklist.

There are TODO's identifying functionality in PreviewsBlackList that
will move to a seperate class (base class) or stay in
PreviewsBlackList (derived class).

Bug: 852593
Change-Id: I684ccb6b1a1b9d0838d34e2db36a8f870aa76fd9
Reviewed-on: https://chromium-review.googlesource.com/1100085
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568130}