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This CL changes the behavior to show dialog one by one,
not showing all of them at once.
This CL also fixes an issue that DownloadFilePicker is not
deleted in some cases.

BUG=868619

Change-Id: I71958b8c8d1383d95996d00c58c84c5523e08bd1
Reviewed-on: https://chromium-review.googlesource.com/1157141
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579955}