clearInitSegments_() {
    const segmentReferences = Array.from(this.segmentPrefetchMap_.keys());
    for (const initSegmentReference of this.initSegmentPrefetchMap_.keys()) {
      // if no segment references this init segment, we should remove it.
      const someReference = segmentReferences.some((segmentReference) => {
        return shaka.media.InitSegmentReference.equal(
            segmentReference.initSegmentReference, initSegmentReference);
      });
      if (!someReference) {
        this.abortPrefetchedSegment_(initSegmentReference);
      }
    }
  }
Remove all init segments that don't have associated segments in
the segment prefetch map.
By default, with delete on get, the init segments should get removed as
they are used. With deleteOnGet set to false, we need to clear them
every so often once the segments that are associated with each init segment
is no longer prefetched.
@private