Add DOM Content interactive duration

[micmro]

There is a domContentLoaded mark in the navigation timing, but it would be good to calculate it relative to domLoading.

ref: https://developers.google.com/web/fundamentals/performance/critical-rendering-path/measure-crp

var t = window.performance.timing,
          interactive = t.domInteractive - t.domLoading,
          dcl = t.domContentLoadedEventStart - t.domLoading,
          complete = t.domComplete - t.domLoading;