Bug 1728749 - Pre-select OS dialog when using Pipewire for screen sharing, r=emilio

Right now when using the OS/portal provided sharing dialog, the
selection in Firefox:
 - does not serve any purpose as the sharing dialog is the only option
 - unnecessarily add two more clicks and potentially confusion

Thus lets pre-select the OS sharing dialog in this case.

Differential Revision: https://phabricator.services.mozilla.com/D124332

browser/actors/WebRTCParent.jsm

@@ -870,6 +870,11 @@ function prompt(aActor, aBrowser, aRequest) {
               );
               item.deviceId = device.id;
               item.mediaSource = type;
+
+              // In this case the OS sharing dialog will be the only option and
+              // can be safely pre-selected.
+              menupopup.parentNode.selectedItem = item;
+              menupopup.parentNode.disabled = true;
               break;
             }
             if (type == "application") {