Invalid screen capturing extension-id cases fixed.

If you linked invalid or 3rd-party extension-id which doesn't have
included your domain in the [content-script
matches](https://github.com/muaz-khan/WebRTC-Experiment/blob/master/Chrome-Extensions/desktopCapture/manifest.json#L16),
then RTCMultiConnection will auto fallback to command-line flag
option.

DetectRTC.screen.extensionid =
'invalid-or-3rd-party-extension-id';