static _getLongestChain(tree) {
    const longest = {
      duration: 0,
      length: 0,
      transferSize: 0,
    };
    CriticalRequestChains._traverse(tree, opts => {
      const duration = opts.chainDuration;
      if (duration > longest.duration) {
        longest.duration = duration;
        longest.transferSize = opts.chainTransferSize;
        longest.length = opts.depth;
      }
    });
    // Always return the longest chain + 1 because the depth is zero indexed.
    longest.length++;
    return longest;
  }
Get stats about the longest initiator chain (as determined by time duration)
@param {LH.Artifacts.CriticalRequestNode} tree
@return {{duration: number, length: number, transferSize: number}}