Awesome

Bubble Mirror Chrome App

A Chrome App to capture your WebCam video feed in a tiny bubble during a screencast recording session on your ChromeBook.

This is easily achieved by combining the Dev Channel only app.window.alpha permission and app.window.alwaysOnTop permission.

And last, but not least, you can choose the Video Source in Context Menu options.

Get it on the Chrome Web Store at https://chrome.google.com/webstore/detail/mdebpbggmcnpmpndgppmljplihbmbamb