[Background fetch] Pass request initiator to download service

The request initiator is required to perform certain security checks,
see |request_initiator| in url_request.mojom for details. Since
background fetch doesn't work like a normal download, we need to pass it
to the download service.

R=peter@chromium.org

Bug: 40058068
Change-Id: I680e759afda3c582d38c05fb98e322065c43f124
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6464959
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1449821}