Enable sessionavailable event when launching with fxr://

[jdm]

The fxr:// handler puts FxR into a kiosk-mode kind of environment that doesn't have a URL bar. It would be good to support automatically entering immersive mode as part of that.

[paulrouget]

So we would set the pref dom.webxr.sessionavailable to true if the app is start from fxr://?

[paulrouget]

Can I have an example that would support sessionavailable to make sure this works as expected?

[jdm]

https://github.com/servo/servo/blob/master/tests/wpt/mozilla/tests/webxr/sessionavailable.html is the only content that exists right now - the sessionavailable event is fired at the navigator.xr object as part of the page load process, so you could modify any other content add a listener for that event and invoke navigator.xr.requestSession.