static getBiggestImpactNodeForShiftEvent(impactedNodes, impactByNodeId) {
    let biggestImpactNodeId;
    let biggestImpactNodeScore = Number.NEGATIVE_INFINITY;
    for (const node of impactedNodes) {
      const impactScore = impactByNodeId.get(node.node_id);
      if (impactScore !== undefined && impactScore > biggestImpactNodeScore) {
        biggestImpactNodeId = node.node_id;
        biggestImpactNodeScore = impactScore;
      }
    }
    return biggestImpactNodeId;
  }
We want to a single representative node to represent the shift, so let's pick
the one with the largest impact (size x distance moved).

@param {LH.Artifacts.TraceImpactedNode[]} impactedNodes
@param {Map<number, number>} impactByNodeId
@return {number|undefined}