onSegmentAppended() {
    this.hadSegmentAppended_ = true;
    if (this.gapJumpTimer_) {
      this.gapJumpTimer_.tickEvery(this.config_.gapJumpTimerTime);
    }
    this.onPollGapJump_();
  }
Called when a segment is appended by StreamingEngine, but not when a clear
is pending. This means StreamingEngine will continue buffering forward from
what is buffered.  So we know about any gaps before the start.