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If we have multiple file picker dialogs, we queue the callbacks in a deque
so that they will be handled sequentially. However, if one of the download
is removed while enqueued, OnConfirmationCallbackComplete() call to
ShowFilePicker() will cause an reentrance into OnConfirmationCallbackComplete().
This CL fixes the issue by posing a task to call ShowFilePicker().
Unit test is also added.

BUG=904319

Change-Id: I00d324a06977a173671a3405594371ccbdc4a5fc
Reviewed-on: https://chromium-review.googlesource.com/c/1332776Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608040}