The meaning of framebufferScaleFactor was just updated in the WebXR explainer (https://github.com/immersive-web/webxr/pull/353) and will soon be reflected in the spec. Chrome needs to update it's implementation to match the new definition, as well as expose the new XRWebGLLayer.getNativeFramebufferScaleFactor function.
Comment 1 by ddorwin@chromium.org
, May 18 2018