Uncaught Error: SECURITY_ERR: DOM Exception 18 when I try to set a cookie
You're most likely using this on a local file over the file:// URI scheme, which cannot have cookies set. Put it on a local server so you can use http://localhost.
I also had this issue while developping on HTML5 in local. I had issues with images and getImageData function. Finally, I discovered one can launch chrome with the --allow-file-access-from-file command switch, that get rid of this protection security. The only thing is that it makes your browser less safe, and you can't have one chrome instance with the flag on and another without the flag.
You can also "fix" this by replacing the image with its inline Base64 representation:
img.src= "data:image/gif;base64,R0lGODlhCwALAIAAAAAA3pn/ZiH5BAEAAAEALAAAAAALAAsAAAIUhA+hkcuO4lmNVindo7qyrIXiGBYAOw==";