Various issues exist with supporting AR and VR session simultaneously
!) either ARCoreDevice or GVRDevice are available depending on the WebXR-hit-test flag
2) the code in XRFrameProvider renders a frame it got back from a magic window callback for all sessions. We need it to only do that for sessions that wanted AR functionality.

This is probably blocked on https://bugs.chromium.org/p/chromium/issues/detail?id=828321 to some degree.