WebAppInstallInfo: Remove invalid states in w_a_data_retriever_unittest

In preparation for removing the default WebAppInstallInfo constructor so
invariants like required fields can be enforced.

This test previously relied upon setting invalid WebAppInstallInfos (eg.
missing manifest_id and start_url) to set mock data. Now changed to use
a unique_ptr so the whole struct is missing in those cases.

POC for this change: dmurph@ if glenrob@ is unavailable.

Bug: b/280862254
Change-Id: I5259170d50dc724a8f003fb8b5389bf2f05d7fa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5518098
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1297269}