add getFirstContentfulPaint()

[John Riviello]

should behave similar to getFirstPaint() with the "first-contentful-paint" name instead of "first-paint"

See https://developer.mozilla.org/en-US/docs/Web/API/PerformancePaintTiming for docs

[Kamran Hamid]

In surfnperf.js file whenever getFirstPaint function is called you want to lookup for first-contentful-paint rather first-paint(which is a current behaviour), right?

[John Riviello]

closed via #85