emit sessiongranted on page load

[michaelybecker]

Context: #1, #2

sessiongranted is the future way browsers will initiate XR sessions, which will also enable automatic session start on page load (without user input, "VR buttons", etc).

XR Packages already implement them, as well as the Oculus browser (and AFrame).

Metachromium currently gets around the button by allowing automatic session request, i.e

navigator.xr.requestSession('immersive-vr', {
 optionalFeatures: [
 'local-floor',
 'bounded-floor',
 ],
}).then(onSessionStarted);

We want MC to conform to future standards by emitting a sessiongranted event on page load, so we can wrap this kind of code in

navigator.xr.addEventListener('sessiongranted', e =>
{
[CODE]
});

[avaer]

This might be easiest to accomplish with a baked-in Metachromium extension that emits this event and is installed into metachromium by default.

Note however that approach requires weird whitelisting flags on the command line to make the extension load in the default profile.

[shu8]

I think we might also be able to do this using the DevTools protocol and injecting a JS snippet to the page to emit the event.

Not sure which method is cleaner to do though!