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also fixes an issue that DownloadDBImpl could be null for incognito downloads

BUG=842245

Change-Id: I1688b6ad15c400a27d6877b60f149c7eee181794
Reviewed-on: https://chromium-review.googlesource.com/1147604
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577992}