_backfillReceiveHeaderStartTiming() {
    // Do nothing if a value is already present!
    if (!this.timing || this.timing.receiveHeadersStart !== undefined) return;

    this.timing.receiveHeadersStart = this.timing.receiveHeadersEnd;
  }
TODO(compat): remove M116.
`timing.receiveHeadersStart` was added recently, and will be in M116. Until then,
set it to receiveHeadersEnd, which is close enough, to allow consumers of NetworkRequest
to use the new field without accounting for this backcompat.