[PWA] Defer browser shutdown during PWA installation

Create a ScopedKeepAlive for the duration of the OS integration portion
of PWA installation.

Installing the native app shim for a progressive web app on macOS can
involve spawning a helper process and performing Mojo IPC with that
process. If that happens right as the browser is about to shut down, the
IO thread that services Mojo IPCs can exit while the PWA installation
thread is waiting on a reply from its helper process. This results in
the shutdown hanging as the main thread waits indefinitely for the
thread pool to become idle and be torn down.

This was easy to reproduce with browser tests in certain build
configurations when ad-hoc signing for PWAs is enabled, but could
theoretically happen in the browser itself if the timing was just right.

The ScopedKeepAlive is not enabled for ChromeOS as it interacts poorly
with browser tests that use `InstallSystemAppsForTesting`. I have filed
https://crbug.com/394384898 to follow up on that.

Fixed: 390734065
Include-Ci-Only-Tests: true
Change-Id: Ie6e58c155bffda9c977682493e45384b4a6c511b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6220602
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Mark Rowe <markrowe@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1418736}