addByteSize(ref, byteSize) {
    const imageData = this._imageCache.get(ref);
    if (!imageData) {
      return; // The image data isn't cached (the limit was reached).
    }
    if (imageData.byteSize) {
      return; // The byte-size has already been set.
    }
    imageData.byteSize = byteSize;
  }
PLEASE NOTE: Must be called *after* the `setData` method.