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As we moved the download id generation logic to DownloadManagerImpl, but
incognito download is still using regular profile's DownloadManagerDelegate
to generate the id. So it is possible that incognito download may have the
same id as a regular download.
In theory this shouldn't be an issue as incognito and regular download belongs
to different DownloadManagerImpl. However, Chrome's download home and extensions
use id to uniquely identify a download. Thus causing the issues when accessing
these downloads from the download home and extensions.

BUG=902336

Change-Id: I3591153e4f453c75d4877f5ce1088c096d3edf1e
Reviewed-on: https://chromium-review.googlesource.com/c/1330214Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607059}