Add a test verifying that AsDebugValue produces some output

Also set a default `base_url` and `seed` in CreateRandomWebAppParams
so they can be omitted from callsites, and removed params from several
callsites.

Per discussion in person around crrev.com/c/4542045

Change-Id: Ic7ef572b388709aa1cc3bd1581eece88cdb59b3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4542262


Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148909}