VisualMetric progress generates null timestamp when Gecko profile is on and windowRecorder is off

[sefeng211]

Sample output

                "ContentfulSpeedIndexProgress": [
                    {
                        "percent": 0,
                        "timestamp": null
                    },
                    {
                        "percent": 81,
                        "timestamp": null
                    },
                    {
                        "percent": 81,
                        "timestamp": null
                    }
                    ......

These null timestamps are not right, and I am fixing them

[soulgalore]

This is still an issue. It's reproducible by: bin/browsertime.js --firefox.geckoProfiler -b firefox https://www.sitespeed.io --video --visualMetrics -n 1