There is no easy way from within an open instance of Chrome. You need to go to the terminal and open a new Chrome instance in application mode, passing the URL.

OSX

/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --app=URL

Windows

Chrome.exe --app=URL

(prepent Chrome.exe with the Chrome folder, which varies by Windows version - see here)

Linux

google-chrome --app=URL