How to disable 3rd party cookies in Chrome?

In the current Chrome (v56, checked 2/23/2017) it is no longer in about:flags, but moved to the main UI.

1. Click on the triple-dots to open the menu (top right)
2. Click Settings
3. Scroll all the way down to the link "Show advanced settings" and click it
4. In the Privacy section click the Content Settings button
5. Under the Cookies section there is a checkbox for Block third-party cookies and site data. Make sure this one is checked to prevent third-party sites from setting cookies.

After enabling this is probably also a good time to delete all existing cookies.

type about:flags

into the url bar and hit enter, then enable "Block all third-party cookies"