Augmented reality model-viewer example doesn't recognize webxr

[jdm]

https://modelviewer.dev/examples/augmented-reality.html

Running with --pref dom.svg.enabled, I get no indication that AR is supported in servo.

Even with a user script containing:

XRSession.prototype.requestHitTestSource = function() { return new Promise(() => {}) };
XRFrame.prototype.getHitTestResults = function() { return []; }

One possible issue is that hit-test is a required feature in this code:

    async resolveARSession(scene) {
        assertIsArCandidate();
        const session = await navigator.xr.requestSession('immersive-ar', {
            requiredFeatures: ['hit-test'],
            optionalFeatures: ['dom-overlay'],
            domOverlay: {
                root: scene.element.shadowRoot.querySelector('div.annotation-container')
            }
        });
        const gl = assertContext(this.renderer.context3D);
        await gl.makeXRCompatible();
        session.updateRenderState({ baseLayer: new XRWebGLLayer(session, gl, { alpha: true }) });
        let waitForXRAnimationFrame = new Promise((resolve, _reject) => {
            session.requestAnimationFrame(() => resolve());
        });
        await waitForXRAnimationFrame;
        scene.element[$onResize](window.screen);
        const { framebuffer, framebufferWidth, framebufferHeight } = session.renderState.baseLayer;
        this.threeRenderer.setFramebuffer(framebuffer);
        this.threeRenderer.setPixelRatio(1);
        this.threeRenderer.setSize(framebufferWidth, framebufferHeight, false);
        return session;
    }

Another issue is the ARGlyph present in https://modelviewer.dev/node_modules/@google/model-viewer/dist/model-viewer.js which is an SVG element, so it's possible that the button is present in the page but it's not rendering in a meaningful way.