Use GetBaseAppId in PWA AUMI.

This prefixes the PWA AppUserModelId with GetBaseAppId(), to distinguish
between different Chrome channels, and different browsers.

Existing chrome proxy shortcuts pinned to the taskbar will get migrated
by MigrateTaskbarPins on startup - GetExpectedAppId looks at the
shortcut commandline and prefixes the expected app id with
install_static::GetBaseAppId() if the command line has a kApp or
kAppId switch.

Bug: 1084751
Change-Id: I91d21069c8313e1e274cbde22ba3d203c288287d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285399


Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798174}