FakeXRDevice should have a param for default framebuffer scale factor

Currently [this WPT test](https://github.com/web-platform-tests/wpt/blob/master/webxr/xrWebGLLayer_framebuffer_scale.https.html#L28) is using a Chromium-specific interface for retrieving the FakeXRDevice's framebuffer scale factor, which itself is just a const in Chromium's `webxr-test.js` polyfill. It should probably be a proper part of the FakeXRDevice interface instead.