Method that will be executed when the image download has completed.
|image_url| is the URL that was downloaded and |http_status_code| is the
resulting HTTP status code. |image| is the resulting image, possibly at
multiple scale factors, or NULL if the download failed.
Method that will be executed when the image download has completed. |image_url| is the URL that was downloaded and |http_status_code| is the resulting HTTP status code. |image| is the resulting image, possibly at multiple scale factors, or NULL if the download failed.