Use SystemWebAppDelegate rather than SystemWebAppManager

SystemWebAppDelegate is exposed from SystemWebAppManager::GetWebApp()
and in AppBrowserController::system_app() and functions can be called
directly from it.

Bug: 1241262
Change-Id: I5f0c00ec4961225ba2545ff1d08cb5d824360944
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3115730


Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Dominic Schulz <dominicschulz@google.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915064}