Use URL from download to check if it is allowed when request initiator is empty

Currently Chrome uses request initiator to check if a download should be
allowed. However, request initiator may be empty for browser initiated
download. This causes DownloadRequestLimiter to use tab's visible
URL to determine whether the download should be allowed. However, for
browser initiated download, the actual URL may be different from the
visible URL. As a result, this CL fixes the issue by using
download's URL to determine if download should be allowed.

Bug: 328278718
Change-Id: I5a5d4a5d09b069ef1bbb8e74ad1630500052236b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5838818
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1352226}