Move url_loader_factory to components/webapps/isolated_web_apps

This CL moves the remaining bit of the URL loading flow for IWAs to
components/. Note that since the IWAURLLoaderFactory holds a reference
to BrowserContext that doesn't support observers (contrary to Profile),
it's also necessary to spin up a new shutdown factory to keep track
of its lifetime.

Bug: 432676258
Change-Id: Idce5cdccc95483a10356514b8ac55c33824875ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6770240
Commit-Queue: Andrew Rayskiy <greengrape@google.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Ryan Sultanem <rsult@google.com>
Cr-Commit-Position: refs/heads/main@{#1491681}