Enabled download home v1 to work with the new backend

This CL contains several fixes:
1 - Download home v1 can work with the new backend. This required ignoring
    the DownloadItem updates in order to avoid duplicate updates with OfflineItem.
2 - Fixed a bug where incognito profile's download provider was getting
    created inadvertently.
3 - Fixed a possible NPE

Bug: 940798
Change-Id: Ia0a91d2ffe329bb77fb58b04f387fd0feaaa5b81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516479
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640148}
6 files changed