chromium: add rtc_use_pipewire

This provides the browser flag #enable-webrtc-pipewire-capturer, which
adds support for screensharing on Wayland via xdg-desktop-portal.

The browser flag is disabled by default until a user enables it. At
least one other major distribution (Arch) enables this compile time
option, and so I believe it should be safe to enable by default.

This is also needed to support xdg-desktop-portal-wlr which was added in
https://github.com/NixOS/nixpkgs/pull/83485.