can do this clamping for very small
      // (e.g. < -0.1) values, accounting only for floating point errors, and
      // fail loudly otherwise.
      total: Math.max(totalBlockedDuration, 0),
      events: {}
    };
  }

  /**
  * Generate a breakdown that attributes where wall clock time goes in
  * |rangeOfInterest| on the renderer thread.
  *
  * @param {!tr.model.Thread} mainThread
  * @param {!tr.b.math.Range} rangeOfInterest
  * @return {Object.<string, Object>} A time breakdown object whose keys ar