Add policy to remove blacklisted extensions.

This adds a policy to allow a Chrome Administrator
to mass remove extensions that they have
previously blacklisted.  Rather than block the
extension from loading at runtime, this will
remove the extension and all potential data from
the machine.

Bug: 844663

Change-Id: I684ed35d1aa9cf8b7a8a1d4b48bc77897fac8eee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534325
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Alexander Nohe <nohe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650074}