static processBFCacheEvent(event) {
    if (event?.notRestoredExplanationsTree) {
      return BFCacheFailures.processBFCacheEventTree(event.notRestoredExplanationsTree);
    }
    return BFCacheFailures.processBFCacheEventList(event?.notRestoredExplanations || []);
  }
@param {LH.Crdp.Page.BackForwardCacheNotUsedEvent|undefined} event
@return {LH.Artifacts.BFCacheFailure}