Replace experimental tabbed window pref with user_display_mode enum

This removes the use of the experimental_tabbed_window_mode web app pref
in favour of using WebApp::user_display_mode by allowing kTabbed as a
value in addition to kStandalone and kBrowser.

This kTabbed only takes effect when the kDesktopPWAsTabStripSettings
feature is enabled, otherwise it is interpreted as kStandalone.

This CL has no changes in behaviour other than the kTabbed state now
syncing across devices (because user_display_mode syncs across devices).
Old versions of Chromium will interpret it as UNSPECIFIED which is
treated as STANDALONE.

Bug: 1241283
Change-Id: I97540e7838de81242bed2f4f73926a04f403b2bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3115707


Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915942}