Lacros: Fix check of source url in Navigate

When deciding whether to reroute navigation to Ash, the Navigate
parameter params->source_contents is inspected. However, this is
done at a point when that value may no longer be the original
incoming value, leading to unexpected behavior. Do the check earlier.

I'm not sure how/if this bug can be triggered at the moment, but a
follow-up CL exposes it.

Bug: chromium:1467718
Change-Id: I8ec482c8c9b88a7f1c9c6d225974edf34168162f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4780057
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1186242}