This looks similar to the Avast Browser Extension, as mentioned on Sitepoint:
Only people who have this extension will see these shields. If I recall, it's not difficult to accidentally install this extension when installing Avast Antivirus, and the Chrome store reports it has "10,000,000+ users". There are other extensions that do the same thing, such as Norton Safe Web and Kaspersky Protection.
There is precious little information I have found out there on what exactly it takes to get the green shield. I do know that users of the plugin can rate your website as safe or unsafe, and likely that is a factor. Many of the websites I remember seeing without the green shield were obscure and likely had few to any ratings.