[PWA]: WebAppRegistrar migration part 2a.

This change is for directory/directories:
chrome/browser/ui/web_applications

In the first (previous) part, a few functions from WebAppRegistrar
were migrated over to IsInstallState, in an effort to flush out
incorrect uses. This CL changes some of those call sites (mostly
tests) to GetInstallSite, which is a more appropriate API
(especially for tests).

This CL was uploaded by git cl split.

R=alancutter@chromium.org

DD: https://docs.google.com/document/d/14JPgYUVX8ECAMdXlp6GwXP-ucXnIIPXjTfxWt-WINLo/edit?usp=sharing
Bug: 379827962
Change-Id: I95190089505f02087cefcd3c3a005c78b804035c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6056269
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Finnur Thorarinsson <finnur@chromium.org>
Auto-Submit: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1389715}