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Add support and UI to disable incognito ntp toggle if that setting is
managed.
EnforcedByPolicy: https://screenshot.googleplex.com/BoEVngjYMsn.png
EnforcedBySettings: https://screenshot.googleplex.com/21UPKQSmc31.png

Bug: 1040091
Change-Id: Ie1f269af20a82025d4ec3d6f92e9c5e8b061f3c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062445
Commit-Queue: Ehimare Okoyomon <eokoyomon@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Lijin Shen <lazzzis@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746581}