Reland "[PWA] Add Chat as preinstalled app, offline support."

This is a reland of commit a979afa35515e91b67bc414b48099e9e4ed03622
To fix this CL, the app factory change was only included on non-CrOS.

Routing to alancutter@ for follow-up here.

Original change's description:
> [PWA] Add Chat as preinstalled app, offline support.
>
> Tested: Manual tests.
> Bug: 376936377
> Change-Id: I2e71ada364d1b6db80090824be856ef3f0b273b8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5996550
> Auto-Submit: Daniel Murphy <dmurph@chromium.org>
> Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
> Commit-Queue: Daniel Murphy <dmurph@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1380019}

Bug: 376936377,378006417
Change-Id: Ibb82619b437bb4a350701c2dfc16de3b85e165b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6005049
Auto-Submit: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1380690}