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Adds the blocking UI to the BVC.
This required a move of the protocol registration to the BVWrangler.

Bug: none
Change-Id: I81d0a868989eee7fea7e8a5d53b03c380e42ee15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547700Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833242}