Sync the chunk cache to storage.

def Flush(self):
    """Sync the chunk cache to storage."""
    if self._dirty:
      self.Set(self.Schema.SIZE(self.size))
      if self.content_last is not None:
        self.Set(self.Schema.CONTENT_LAST, self.content_last)

    # Flushing the cache will write all chunks to the blob store.
    self.chunk_cache.Flush()
    super(AFF4ImageBase, self).Flush()