Enhance WebXR-internal page

Provide real time diagnostic information to chrome://webxr-internals/ (e.g. real-time logs), as well as high-level performance metrics (live framerate, tracking dropped frames, etc.). Some of this could be extended into additional metrics or similar that can be used to monitor general health of webpages, and there is a need to implement these performance metrics as a potential feedback mechanism for the Google Cardboard VR runtime.