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This is because users needs to explicitly know that they can reject
the authentication - tapping outside dialog to cancel is not clear
enough.

Bug: b:156258540, b:144861739
Change-Id: If47cd9bb76a48e6149eba3606cc1434db53662c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2525855Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825477}