akdown that attributes where CPU time goes in
  * |rangeOfInterest| on the renderer thread.
  *
  * Due to approximations, it is possible for breakdowns to not add up to total
  * CPU time in |rangeOfInterest|. Note that |rangeOfInterest| is a range of
  * wall times, not CPU time.
  *
  * @param {!tr.model.Thread} mainThread
  * @param {!tr.b.math.Range} rangeOfInterest
  * @return {Object.<string, Object>} A time breakdown object whose keys are
  * Chrome userfriendly titles & values are an object that sh