Build a cache for our chunks.

def Initialize(self):
    """Build a cache for our chunks."""
    super(AFF4ImageBase, self).Initialize()
    self.offset = 0
    # A cache for segments.
    self.chunk_cache = ChunkCache(self._WriteChunk, 100)

    if "r" in self.mode:
      self.size = int(self.Get(self.Schema.SIZE))
      # pylint: disable=protected-access
      self.chunksize = int(self.Get(self.Schema._CHUNKSIZE))
      # pylint: enable=protected-access
      self.content_last = self.Get(self.Schema.CONTENT_LAST)
    else:
      self.size = 0
      self.content_last = None