This capability, and a couple of other small enhancements, are to be found in the version 3.2 update to the Google Camera. Once you launch a video, the shutter button slides to the right for taking pictures. Several other options have also moved to a slide-out menu on the left, such as Photo Sphere or Lens Blur.
It appears this could be only for Nexus devices, however: I was able to snag the 3.2 update from the Play Store on a Nexus 5X. However, the app is nowhere to be found if you search on a non-Nexus device.
This sounds like a similar strategy to what Google has done with its Phone and Contacts apps, which are built specifically for Nexus phones. Putting them in the Play Store allows for faster, easier updates.
You can try your hand at this right now if you want by grabbing the update from APK Mirror.
The impact on you: If you have a Nexus device, this is a nice update. It also is an indicator that Google is taking a renewed interest in its camera application, which had gone dormant for a while. There’s still some catching up to do in comparison to what other handset makers have produced, particularly all the capabilities in photo-friendly phones like the Galaxy S7 and S7 Edge.