dpwa: Add WebAppInstallManagerTest browser test for multiple sources issue.

This is extra test for this fix:
https://chromium-review.googlesource.com/c/chromium/src/+/3097263

The same web app can be installed and uninstalled by Arc and
the user (sync).

kWebAppStore source provides an offline manifest fields,
which may represent a subset or out-of-date
fields from the real web site's manifest.

Adding a test which simulates how we install same web app from sync over
existing web app from ARC.

Bug: 1238622
Change-Id: I945672743164733b11bc29cc4e2f4a023069566a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3143836


Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918726}