WebApp: Fix SystemWebApp tests.

WaitForExtensionInstalled relies on a random extension added to
the registry. It was a final install step in the legacy system.
In the new system that's not a final sync step.

on_apps_synchronized() is important.

SystemWebAppManagerBrowserTest.Install
SystemWebAppNonClientFrameViewBrowserTest.HideHostedAppButtonContainer
crash when we enable kDesktopPWAsUnifiedInstall:
https://ci.chromium.org/p/chromium/builders/try/linux-rel/101457

TBR=bsep@chromium.org

Bug: 915043
Change-Id: If7650b7f9f86079a4b424f43ff0d811412a77815
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1630372
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663494}