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This allows a client to block signin in incognito.

Bug: 1138458
Change-Id: I724a4dace1cdb6fc1c38f0640c344a12d4482021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2470467
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817087}