[lifetime] Fix and enable TestWithDownloadsFromDifferentProfiles

In removing WebContents::GetURL, GetLastCommittedURL was used, which
broke tests.

In this change, the about:blank url was checked after a new browser was opened,
in which case, there is no committed url. In this case, GetVisibleURL should be
used.

By using GetVisibleURL, there is no change in behaviour from before the breaking
change.

Bug: 1287462
Change-Id: I54c31ba034a8cd9be02d037e4dc82493d4733bd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3389657
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#960515}