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If a download is resumed and the server immediately returns an error
response that requires the download to restart, |download_file_|
is not created.
As a result, resuming the download will not clear up the current
path, and causing the DCHECK to fail.

BUG=982819

Change-Id: Ibe0832e79ca80bd2e76deba454dab2c3d8cb2be1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1706781
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678536}