crdownload files are generated by Google Chrome as a placeholder/partial safe during a download in progress and after it is finished, the extension is changed to the original file. Simply put, in case you were downloading some ISO file through Chrome web browser and the download was somehow interrupted, it might happen that the file is corrupted and you won't be able to continue the download.
No crdownload to iso conversion will solved these issues, you will have to start the download again and finish it successfully.
In a word, direct crdownload to iso conversion is not possible.
So don't expect any so-called crdownload to iso converter or crdownload to iso free online conversion tool available.