Fix it: Using USB Webcam with Google Chromebook

 To use a USB Webcam with Google Chromebook, you first have to enable it.  Go to settings and then search for "camera."  Click on "Site settings" as shown.  You can also get there by going to chrome://settings/content#content or chrome://settings/content/camera (direct link).

Then scroll down to find "Camera."

Change from "Integrated webcam" or "front camera user-facing" to the Webcam you're using.  In my case, it's the Logitech C920, which is labeled as "HD Pro Webcam C920."