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This is to indicate to the user that this is an OS dialog.

Bug: b/156258540
Change-Id: I0c3b4f53682e348a1623e1f812c5991f58134468
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511754
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823723}