Introduces enable_playlist (defaults to !is_brave_origin_branded),
following the same pattern as enable_brave_wallet and enable_ai_chat,
to gate all Playlist code from compilation on origin-branded desktop
builds. enable_playlist_webui now chains through enable_playlist.
Changes are scoped to desktop: iOS and Android don't support
is_brave_origin_branded=true (enforced by asserts in
//brave/components/brave_origin/buildflags/buildflags.gni), so they
have no guards.
Brian R. Bondy
fe1a9add88