On OS X, to open a new Chrome window - without having to close the already open windows first - pass in the additional -n flag. Make sure to specify empty string for data-dir (necessary for newer versions of Chrome, like v50 something+).
open -na /Applications/Google\ Chrome.app/ --args --disable-web-security --user-data-dir=""
I found that using Chrome 60+ on Mac OS X Sierra, the above command no longer worked, but a slight modification does:
open -n -a /Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --args --user-data-dir="/tmp/chrome_dev_sess_1" --disable-web-security
The data directory path is important. Even if you're standing in your home directory when issuing the command, you can't simply refer to a local directory. It needs to be an absolute path.