Add Chrome's firstContentfulPaint user-timing metric (so we can correlate with Firefox's timeToContentfulPaint metric)

Although we have metrics for First Contentful Paint for Firefox (Nightly), we're missing them for both Chrome release + Canary builds.

We don't seem to need to enable a pref for it, so we should be able to just grab values from:

chromeUserTiming.firstContentfulPaint - see https://wpt.stage.mozaws.net/result/190122_BR_13W/?f=json

There are actually a few raw values available:

firstContentfulPaint PerformancePaintTiming.first-contentful-paint (doesn't appear to round up/down)

We can also see in the 2nd screenshot that our Fx-specific raw metric timeToContentfulPaint comes up null in Chrome, which makes sense.