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This CL adds a checkbox that will display the current state of the
notifications prompt quiet UI, which the user can then enable/disable
at will.

The quiet UI can be automatically-enabled by embargo-logic.

Bug: 1014874
Change-Id: Ia5e68877acf4b4dce82fe4eb1679706c62c7f9de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930820Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720430}