[WebXR] Fix closure compiler mangling XRSession.enabledFeatures

[dsnopek]

Currently, when running a WebXR app with a release build using the Closure compiler, it'll crash on entering an XR session with this error (from Chrome with the WebXR API Emulator):

index.js:7340 Uncaught TypeError: undefined is not iterable (cannot read property Symbol(Symbol.iterator))
    at Function.from (<anonymous>)
    at index.js:7340:42

This was introduced by PR #88411, which added a reference to XRSession.enabledFeatures (among other things), but I forgot to add it to the webxr.extern.js file for the Closure compiler, in order to prevent it from doing its usual name mangling thing.

[dsnopek]

Ack, I found another issue related to enabledFeatures: it's not supported on Firefox. It isn't related to the Closure compiler piece, but I could perhaps sneak it into this PR?

[dsnopek]

Nevermind, Firefox needs more fixes than just checking that enabledFeatures is available - I'll work on that in another PR.

[dsnopek]

Nevermind my nevermind :-)

The issues I'm having with WebXR on Firefox appear to be specific to Linux with the proprietary nvidia drivers! See https://bugzilla.mozilla.org/show_bug.cgi?id=1896611

So, I've snuck in the check to make sure that enabledFeatures is available before trying to access it, which should fix Firefox.

[Malcolmnixon]

This looks good, and I can confirm it WebXR is functional in both desktop browsers (with a WebXR plugin) and the Quest VR headset browser.

[akien-mga]

Thanks!